When I was working on all types of guns I spent a fair amount of time prior to deer season, just sighting in people's rifles etc. I found that a big target at just 25 yards and another more normal sized target at 100 was the simplest and fastest way of getting on paper with 1 shot and fine tuning at 100. I simply fired one shot at the big 25 yd target, adjusted the crosshairs to that hole minus a little vertical..then went straight to 100. It was just one shot vs boresighting by eye and hoping it was on paper. So during that time of year, I kept a big 25yd target up in addition to my 100 and 200 targets. I think that's the most foolproof way of doing it but yes, eyeballing down the bore usually works. This way was virtually 100% though without any guessing. Even the collimator style boresighters would lie at times on factory rifles for numerous reasons.