Using Mil Dots to Figure distance
Mildots are a device separate from "Kentucky windage" as mentions.
To use mildots to calculate windage compensation, you first need to use them for their primary purpose which is rage finding. If you know the height or width of your target, then use the following formula:
H is height or width
N is number of mildots
D is distance
An object one yard in height or width will be one mildot in size at 1000 yards.
This means that at 1000 yards, a typical man would be right about two mildots in size, so a mildot to either the left or the right is one yard of windage. Calculating your windage adjustment is something that can change based on ballistics, bullet weight, wind speed, even atmospheric conditions. It takes more practice than math to be practical.
From your question, though, it appears as if you are just trying to compensate for drop. I'm not really sure what resources are available for air rifle ballistics; cartridge users can just look up ballistic charts that give you the basic drops at sea level for a given caliber, weight, and velocity. Or they use online calculators for different atmospheric conditions.
Given your calculations, you either shoot pellets with a very good ballistic coefficient, or are running very high velocities. 1800 FPS was what got me that drop with a .177 and a 20 yard zero. From my limited knowledge of air rifles, that should be almost a $600 gun. That is on the very, very high end of .22LR, more like a .22 Magnum.
Anyways, how to use the mildot...
A mildot is 36 inches at 36, 000 inches. So, at 1440 inches (40 yards), an inch would be .69 mildots. Or 1/N*1000=1440.
That means you should put your target a little more than half way between your crosshairs and the first bottom mildot.
The thing with mildots, is they were meant for engagements at greater distances with ammunition with superior ballistics. For your purposes, this is like measuring your bathroom with a laser range finder. Or racing your child's bicycle around the block in a Lemans GT car. The bicycle might actually win because of how better suited it is for the task.
Consider getting a serious air rifle scope that doesn't have mildot.
Depending on what your ballistics actually are, you may consider a BDC reticle scope from Nikon suited for the .22LR. They also have one for crossbows. And using their SpotOn web app, you can enter your ballistics and print out a cheat sheet that gives your drop values for the ticks on your specific scope. It is a very cool system, and I have always had a good experience with Nikon.
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