Percussion caps are remarkably non-standard. Some one published a list of caps on Muzzle Loader Forum a few years ago, 10's and 11's with micrometer readings. I've put some nipples in a drill and turned them down on a sharpening stone, which is pretty much what you have done.
It wouldn't hurt to pick up a spare pair of nipples. You will need to know the size and pitch of the threads in your breech. You can take a nipple to your hardware store, which may possess a thread measure. If not you can try the nipple in various nuts in the parts section, until you find one that works well. As noted abve, Track of the Wolf has an impressive selection of nipples, including various oversizes, should the threads in the brecch be worn.
