Steve,
Air rifles aren't rimfire mate.
It's all well and good and works on the same principle but with a 40yd zero theres s lot more factors to be added into the equation, the weight of the pellets, the power and conditions all come into play, for me the closer you can get to the quarry the "better" chance of a clean kill and the fieldcraft for me all adds to the experience.
If you enter the relevant information into ballistic software then it will give you "an idea" of the best ranges for rifle power & pellet weight also going off the mag of your scope
Darryl