Drill bits can be used, you just have to do some math. If you get lucky the drill by itself comes out to the size you need. If it doesn't use a set of feeler guages from your "well equipped home shop". Use some snips to narrow the thinnest leaves down to about 1/4" wide. Hit them with the dremel to bevel the edges and remove any burrs from the snips. Now install the largest "too small" dril in the receiver and slide the thinnest feeler into the extractor slot between the boltface and the headspace guage. Depending on receiver contour you may have to snip off the tip and use it as a small shim, some you just let the leaf flex out of the way. Test lockup effort, repeat with thicker feelers as required. Once proper lockup is achieved, add drill and feeler measurements. That's the "missing" guage rod size.
Drill bit "book" sizes:
Measure the actual drills you are using though, the cheap Chinese drills are especially variable in size. A couple different sets might even help to fill in some of those missing sizes. If you are uncomfortable using the WECSOG method, use the measured drill bit as a starting point then purchase only the rods in the size range needed instead of the whole set.
BTW, you can also use the feelers to check the actual headspace measurement on an assembled rifle too.