They mean that if you have 1/4th inch cast-off in the comb line you want to mark the same amount of cast off, or for some slightly more, on the toe line. That way the butt stays vertical but just nudged over a scoche. Some folks like 3/8ths inch other like 1/4th inch and others like 1/8th inch for cast off. It depends on the length of pull that the shooter likes too. When I build a rifle for some one I whittle the stock down to almost finished dimensions and then I have them over to try the rifle on. The close their eyes and mount the rifle. then open their eyes and find out where the rear sight is relative to the front. then I shave off what needs shaved so that they can hit with their eyes closed. It takes a while but its worth it if you are a serious shooter.
Best Regards,
John Cholin