I've not shot a hog in the head with an air rifle, so take my comments with several grains of salt. I have however, done limited testing with pellet penetration on multiple small/medium sized species (testing involves taking a dispatched animal with no head trauma and then seeing if a pellet at a certain speed penetrated the brain cavity and if so the amount of damage it did to generate FPE data). Having disclosed this, there are three important points I've found that need to be considered, 1) FPE @ POI, 2) maximum accuracy/precision distance and 3) pellet construction.
Starting backwards, you need to make sure you're using a pellet that will penetrate the brain case at low velocities. I've come to realize I need to say brain case as the skull is much larger and can allow for misinterpretation of the brain case location for some species as brains come in different shapes and sizes. A great guide book is Animal Skulls by Mark Elbroch as he includes multiple pictures, diagrams, views and dimensions that can be used to help figure out where the shot needs to be on each animal. In terms of the pellets themselves, I've had some major surprises with what actually penetrates versus what doesn't. Per most pellet manufacturers, H&N pellets are the hardest and may be a better option when dealing with thick hide, lots of muscle mass, and bone than another brand but that is up to the shooter to decide. Also, you need to look at how the pellet is designed to deliver energy to the target. Diabolo and cylinder style pellets, especially non lead ones, will usually deliver the most penetration, followed by certain hollow points and semi-wadcutters. Remember that pellet weight and feet per second determine the FPE @ POI requirement, but the style of pellet determines how the pellet functions once it hits the target.
Next is the gun/pellet maximum accuracy/precision distance. For this, you need to know how big the brain cavity is and how big of a lethal zone you want to allow and then find the maximum distance you can achieve it. This will become the maximum accuracy/precision distance for that gun/pellet combination. Some may have the skill to do 10 shots in a 2" circle at 100 yards regardless of winds or other conditions, but I'm not one of them. Each shooter will have their own skill level so what my maximum distance is can be totally different (closer or longer) than another. Bear in mind that this distance can change when different guns, power settings, air pressure, pellet selection, pellet seating and I'm sure some others not coming to mind are included into the equation.
Lastly, we get to the FPE @ POI. It bears repeating, especially for new shooters reading this to understand that what the gun produces at the muzzle isn't as important as the foot pounds of energy (FPE) at the point of impact (POI). So while the FPE rating is important to know (i.e., a gun performing at 18 FPE will never be able to achieve anything higher so it shouldn't be used if 20 FPE @ POI is required) it isn't the only thing that should be looked at. If you can hit the kill zone, but the pellet doesn't penetrate causing a lethal shot, it doesn't matter if you had enough FPE @ POI or not. If you can't hit the kill zone at the distance the animal is located, it doesn't matter if your gun/pellet combination has more than enough FPE for that distance since it won't die if it isn't hit. Rather, what is important is figuring out the maximum FPE @ POI distance with your setup and then seeing if you can duplicate it.
To put this all together, whether it is correct or not, the number I keep seeing for a lethal brain cavity shot on hogs is 30 FPE. Again, I'm not saying this is correct, I just want to use this number for this discussion (I also am not going to get in the discussion if this number is different with front, side, or back of head placement). If the rifle/pellet setup allows 30 FPE at 75 yards (you can get a close approximation from programs like Chairgun Pro), then the maximum FPE @ POI distance would be 75 yards. Now, the second question becomes how big of a kill zone is required for an ethical side head shot.
While the previously mentioned book doesn't list brain case measurements for wild hogs, it does list that the average adult wild hog skull will be around 12 inches long. By looking at the images provided (and the mounted hog hanging over my head as I type this) I can see that the brain casing is high and towards the top so an estimated target size would be about 1 1/2 inches high and 2 inches wide. This can then be dropped down to a 1 inch circle or 1 x 1 inch box kill zone to allow for some leeway if the pellet travels slightly off target. If the gun/pellet setup is capable of putting 10 shots from the expected hunting position (prone w/ bipod, off hand, off hand with sticks, kneeling, etc.) into that 1 inch kill target at a maximum distance of 25 yards, then the maximum accuracy/precision distance is 25 yards even though it is generating much more FPE than the required 30 FPE at that distance. Likewise, if the setup is capable of doing this at 100 yards, the maximum accuracy/precision distance will still be 75 yards as there isn't enough energy in the pellet once it passes this point for a quick dispatch. It is also important to note that I'm qualifying this by saying "expected hunting position." Just because the gun/pellet combination is capable of achieving the 10 shot goal at 75 yards when using a table or placed in a shooting sled, it doesn't mean that you have a 75 yard limit. In almost all cases, the shooting system will always be able to out perform the shooter. Additionally, I'm also going off of absolute minimums with this example instead of wanting a higher FPE (like making the maximum FPE@POI 40 FPE versus 30 FPE) in case you shoot a hog that didn't read the same scientific materials you did and doesn't know how it's supposed to act.
Lastly, we need to ensure that our chosen pellet has been designed to travel at least 3 inches after impact so it can enter the brain case dispatching the hog and not stop short for any reason. Of course, the best way to test this is to shoot a hog in the skull with different pellets at different velocities, but that isn't always possible. Other methods of testing can include shooting through different thicknesses of plywood with ballistic gel or some other medium behind it to catch the pellet so depth of penetration can be measured. For this method, I'd recommend starting with 1/4" plywood and ballistic gel or plastalina modeling clay. While some will say clay won't represent hide, tissue and bone properly neither will plywood or ballistic gel. What's important is that you use the same materials each time so you can see what is giving the best results. If the 1/4" plywood stops the pellet at the maximum/precision distance, you need to either choose a different pellet and/or move the wood closer until it doesn't. On the other hand, if the pellet goes straight through the wood and has decent penetration into the gel/clay then try 1/2 inch plywood and see what happens. Again, this isn't best case scenarios, but it will at least show you something about how the pellets are performing and may show that certain gun/pellet combinations are more suited for brain shots while others are better for heart/lung shots.