I have access to 25 yd range, get it on paper, hard to do at 100 without expending more ammo, been there done that. I've used the 25yd zero on my scopes for years, then check at 100 have always been good. Have also read where certain units the zero may change depending on mode selected? Seems to be issue with some units, haven't heard that about NVision.You can use alum foil or foil tape, just angle it slighrly up toward the sky seems to make it a little better. Hamd warmers work good. I've also used an ice cube in a ziploc bag. All of the above worked great. As for 25 yard sight in, sight it in at 100 yds or more and be done with it and know where you will hit. I used to see all of that crap on thermal groups on fb. "I sighted in at 50 (or 25) and I miss at 100 or 150 yds, what's wrong...." I guess 25 yds is fine if you want to walk up and shoot pigs in the head from 10-30 yds away, which is fun.