The R9/HW85 is my favorite springer and I have several, all of which have been home tuned. You will likely get differing opinions on this but this is mine.
First of all, deburr all the slots and holes taking precautions to keep metal shavings from staying in the compression tube. You can measure the ID of the compression tube and the seal at different points to get an idea of what you are working with but there can be variations in ID of the tube itself from the rear where the slots and holes are to the area in the tube in front of the cocking slot. The reasons for variance in this area can be from the machining of the slots and holes to an over tightened scope mount.
My goal is to have the seal fit tight enough that takes approximately 4-5lbs of pressure to move the piston once the seal and piston is deeper in the tube. I used a food scale to get an idea of what that feels like.
After applying a small amount of lube on the front part of the seal -- Install the piston with seal and see how tight the fit is. I go slow at this point keeping an eye on the seal as it passes the slots and holes and if needed use a tool to help the seal pass those points.
If a seal is too loose you can get a harsh firing cycle and if too tight it will reduce the velocity. The design of the seal and the material it is made from will affect how the fit to compression tube feels. If it is too tight you need to size it down but if it falls in with only the weight of the piston when the tube is held vertically I consider that far too loose. I have seen comments (opinions) made by others that say that's OK so it's up to you to decide what's good for your needs.
If resizing a seal, be aware of where you reduce it at. A parachute design with a wiper band will probably need the wiper band targeted as the main area because the grove that allows the parachute to conform to the tube does allow give at the front of the seal. I use sandpaper on a block while the piston is rotated. Go slow, measure for your information, and try fitting often so you don't go too far.
A new seal will break in with use and if it's overly tight it can wear in over time but I have never gone that route.
This may or may not help you but being able to home tune your R9 will give you confidence and pride in what you have done.
