This assumes you are basing it on an accurate 100 yard zero and just shooting groups to record total drop in inches first, which happens to be the best way to get an accurate BC too.
1 moa is equal to 1.0467” at 100 yards, so that inch figure will double at two hundred yards, triple at three hundred and so on because it is a measurement of angle.
Example; 1 moa at 200 = 2.093”… 1 moa at 300 = 3.14” and so on.
If your bullet drop at 500 yards is 49.4”, then 9.4 moa is the correction needed.
moa correction needed = D /M / (R / C)
Where
D = Drop in inches
M = MOA in inches (1.047”)
R = Range
C = Constant (100)
49.4 / 1.0467 / (Range in yards / 100) = 9.439 moa correction needed
For all practical purposes, I move the decimal two places to the left on the range distance and divide this into the number of inches of drop and multiply by .955, this will be real close if your scope is calibrated to true moa.
Using a chronograph and a 4’x8’ sheet of plywood standing on end using the top edge as an aim point will help you get a drop chart out to 600 or 700 yards depending on the MV and BC. When you are all done you have the data to get a drop chart from any program to go out farther. If you know the average velocity for each group shot, you will be able to modify the BC on the program to make the trajectory fit your data, this BC is the one you should use. Take good records of the temp and elevation especially, BP and humidity are somewhat less important, humidity especially.
I like to do this at 20-30 degree temperature swings to record velocity drop along with new zero data too. Velocity at different temps will be very important if your powder is not too stable, which a lot of them are not.
The important thing to establish and record for a drop chart is what moa "your" scope indicates when zeroed at each range interval you've went back and verified using the method of dialing it in. Sometimes this takes some time to nail down here.