Actually the sky won't fall if the nipple hole is a little too big. If the hammer comes back, then the hole is way too big, (or your load too hot) and you go back down to a smaller size. The hammer won't come flying off the gun or anything. It's not a big deal. Have you ever looked at the hole in a musket nipple? It's very BIG.
Another solution is to vent the drum, or bolster with a very small hole. The gasses and crud will then vent out there and not out the nipple, and keep the flash channel clean. Obviously you want that hole to point forward, or up and forward.