"...at 25..." Ha! Why would you sight in at 25 yards?
A one shot sight in is ludicrous. "need to have your rifle setup in a shooting vise" That being the key. I'd have called any of ya'll you names if you had told me this, but Crappy Tire sells one for $50. No provison for dealing with recoil though.
http://www.canadiantire.ca/AST/brow...se/CROSSSELL~0755426 Bore+Snake.jsp?locale=en
"...resources for the more math inclined folk...has to be a trig function..." It isn't a math thing. Trig is far more complicated than necessary too. Bullets don't fly in straight lines anyway. They fly in an arc. When the bullet leaves the barrel it rises then comes back down. The principle of sighting in and ballistics is to know where the bullet is at a given distance. The math involved includes initial velocity, its loss over a distance, bullet weight, bullet shape, the effect of gravity, etc.
The 2 shot sight in method, requires a very solid rest. Usually an expensive mechanical device. Fire one shot, then adjust the reticle to the hole, if you have a spotting scope that'll let you see the hole. Saw it in a gun rag about 20 years ago. Doesn't work with a shooter in the equation.
For normal people, nerds or not, you need a group with whatever ammo/bullet weight, off a solid bench rest first. Then, off a solid bench rest, you adjust the sights for the POI to hit the bull at 100. The key is the rest. No bipods. Sight in off a bench then go to a bipod will change the POI.
Sighting in is about moving the group where you want it. Change ammo or bullet weight and you get to do it again.
"...I've been doing it wrong for over 40 years..." Ain't it daft?