I guess he misread the wind by a 12" miss.
There is your problem. Two problems, actually.
#1 you don't "guess"
#2 inches have nothing to do with shooting rifles at distance
To correct #1 you measure the difference in POA and POI with your reticle. To correct #2 you tell the shooter the number you determined in #1. Simple. No math required. You (and most LR shooters learning the ropes) are making it more difficult than it has to be. When it's time to shoot your rifle, leave the inches in the truck.
NOTHING is done in inches or any other linear measurement.
Make a dope card in MILS.
Dial/hold in MILS according to your dope card.
Observe & correct (if needed) in MILS
Too easy.
Oh, and on the 5" zero correction @ 300 yards question.... If you can't see bullet holes to measure in mils you can't see them to measure in MOA. When you go to the target you mark the holes (dark permanent marker into the group center on a light target and vice-versa) and measure in MILS when you get back to your firing point. That avoids math altogether.
If you just have to do math for some reason (that's just craziness, btw
), keep in mind that there are 2.5 CM (2.54 but we don't need to worry about the 0.04) in one inch if you must measure your required correction in inches.
5" * 2.5 = 12CM
0.1 MIL (one "click" on most MIL scopes) works just like the one inch per 100 yard MOA thing does except it's 1CM per 100M. This is why people sometimes refer to MILS as "Metric" even though that's not correct.
12/3=4 (0.4 MILS correction) Again, too easy.
If you need to convert yards/meters just remember meters + 10% for yards and yards - 10% for distance in meters. That's close enough to get you on target waaaay out there.
Examples:
1000m +10% = 1100 yards (within 7 yards from the actual number)
1000y - 10% = 900 meters (within 14 yards from the actual number)
I was a 100% MOA shooter many years ago. Made the switch to MILS and immediately saw the advantages. I finally sold my last MOA scope last week and will never go back. I would much rather deal with 6.9 MILS than 29.22 MOA.
In the end, either system works. MILS are just a little simpler once you let go of linear measurements.
