I think the only accurate way to do this is to shoot paper at the ranges you are interested in.
Chairgun is good, but only as good as the data you put in.

For example my TX200 holdovers wouldn't match up with anything chairgun said UNLESS the ballistic coefficient of the pellet was changed. That led to some googling which is when I found out the ballistic coefficient of the pellet is determined by the power plant (spring/PCP/CO2) and to a lesser extent the individual barrel on your airgun. So the default values don't always work.

Having said that, Chairgun is really good for testing adjustments to scope height or zero range etc. Provided you have correlated the chairgun numbers, with actual tested numbers, first.

In term's of carrying out the test, Charub's link is perfect if you want to use true mil-dot magnification (usually 10x). If you want to use a different magnification, you will need to adjust the size of the targets.