Original longrifles had very slender forends -- the upper edges, along the side flats of the barrel, were almost like knife blades. A common tendency (I'll stop short of calling it a mistake) of new builders is to want the wood in this area to be a little thicker. Even knowing it should be fairly slender, they're uneasy about removing that last 1/16" or so of wood. Most commercial nosecaps, though, are made to fit rifles that follow traditional styling, meaning their upper edges just about touch the barrel. You're right about the amount of wood that this leaves beneath them -- there isn't much, and it's extremely fragile, particularly if you're using higher-grade curly maple. You have three options: spread the entire nosecap just a hair (not just the rear portion, but the half-octagon area that fits around the muzzle), which will allow you to leave a tiny bit more wood but will also ruin the tight fit of the cap to the barrel; grit your teeth, make sure your tools are sharp, and, using a good metal file to finish, very gently remove the last of the wood necessary to make the cap fit as it should; or make your own nosecap of sheet brass, which is relatively easy to do but sometimes not so easy to get just right. This would allow you to provide for a little extra wood beneath it, while still having it mate up to the end of the barrel.