Scope zero distance?

Depends on my gun and distance. Some guys use clicks, I use mildots and zero where the pellet is traveling at its highest point. Both my airguns this is 30 yards. This way I have no hold under, I find having to remember just hold over (bottom half of the scope) easier and less confusing. My long distance 30 cal I use height adjustable rings. Zero is 65 - 75 yards(still making adjustments) so the scope isnt maxed out when shooting 100+ yards.

 
Chairgun has a function that you can set your kill zone. For example with my 22 cal shooting 18gr pellets at 900 fps I set mines to 1". What that means is that when I set my short zero to 18 yards my long zero will be 55-56 yards. So between these two distances I can aim zero on the the POI will be no more than 1/2 inch high or 1/2 low. Of course you can still figure out holdover outside these distances but it remove the need to figure out POI for your set kill zone.
 
37 yards to be precise. I have found this to be the best yardage that works for me. Using a Vulcan and an MTC Viper Connect 3-12 with an SCB 2 reticle, I can take targets ranging from 30 to 160 yards which is the point at which I run out of Mil Dots. This is at 10 power which is what the scope is true at. I can also using the 37 yard zero take targets from 10-30 yards as well just by holding over. No need to hold under. Makes it easy to remember since I don't need to memorize both. 
 
Like Sam63, I find my bullets trajectory peak and make that my zero. Everything from there either further out or closer in are clicks back up on my elevation turret. I use the turrets for windage/elevation and put cross hairs directly on target, but one could hold over for everything as well.

BTW, the elevation turret label is marked out 7-68 yards in 1 yard increments and 0 - 120 clicks for use with Strelok Pro.