What is the formula sir?
Your ENY plane / the ENY of the plane you kill = earned perks. This of course varies depending on the perk point bonus as well as if you land successfully, ditch, etc.
IE: Your plane is a Spit IX which is 20ENY. Their plane is a P-51D, a 5 ENY.
20 / 5 = 4. You would have earned 4 perks just for this kill.
If memory servers correct, you would multiply the earned perks with the perk point bonus to get your total perks for if you land successfully.