When he's 10 inches left and 9 inches low at 780 yards you can do one of the following:
- Realize that you have no fucking idea what 10 inches looks like at 780 yards and give some random best guess that he then gets to do math with to try and figure out what the adjustment should be
OR
- Use the calibrated ruler that's in-front of your eye and tell him exactly, in the same measurements his scope is in, what his correction is without having to do any math on either your or his part
You don't need to know his ballistics; trhe correction is the correction on his current ballistics, assuming you're both using the same system of MOA or MILS. 1 MIL still equals 1MIL regardless of the distance. Nothing is as fucking useless as a spotter telling me shit like 'hit below the target a bit' or 'it went to the right of the plate'.