Objective is to coordinate the line of sight through the optic with the ballistic path of the cartridge at the zeroing target range.
Variables are: Environment influenced by temperature, barometric pressure, altitude and humidity; Cartridge ballistics specified by Ballistic Coefficient (BC) and Muzzle Velocity (MV) plus consistency of those values; Optic height above the barrel plus any type of inclined optic base; and to some extent the age and condition of the firearm. Best to zero your firearm close to the conditions in which you plan to use it.
There is a lot of discussion whether it’s best to zero at 100 or 200 yards. A popular comment is that 200 yards is closer to the usual target range so better to zero at 200 yards. The range you zero at doesn’t change the bullet path too much. A 30 Caliber cartridge with a 100 yard zero will be about 13” low at 300 yards vs. 8” low with a 200 yard zero. You are probably going to make an MOA adjustment at 300 yards either way. With that in mind, more convenience and precision zeroing at 100 yards seems like the preferred choice. Most ranges have more 100yd benches and even a 10x scope can locate holes in paper targets at 100 yards.
The basic plan is to get on paper quickly at a short distance, make MOA corrections, and then get on paper at the selected Zero Range. Then make final MOA adjustments and test to confirm optic is zeroed.
One way is to boresight at a long unknown distance so that the optic is nearly horizontal to the barrel. Knowing that, ballistic mathematics can adjust the optic MOA to be near zero at the target range. This method is diagrammed on the Zeroing Page.
A second way is to bore sight at a short known distance, then do a test shot to measure actual optic position and then make MOA adjustments to be near zero at the target range. If your plan is to zero at 100 yards, you should be about 1” low at 25 yards for 30 caliber cartridges. See the diagram below. Only the 0.22 Long is significantly different and rarely used at longer distances.
MOA Adjustments: 1 MOA = 1.047” at 100 yards and is a linear relationship. So 1 MOA = 0.26” at 25 yards and 2.1” at 200 yards.