That's a really good idea! It's funny, because I was just thinking about this dilemma this week. I've been lampworking for 10 years and still can't seem to make donut beads that match.
Then I remembered that I had bought these donut bead shapers from Howaco glass a while back:
http://www.howacoglass.com/tools_shaping.html
I tried them a couple of days ago and I am in love! I can make donut beads just fine, but they would end up all different sizes. Now I just have to roll them in the right size cavity and check the size that way.
Your method is great, though, and it doesn't require any tools except a ruler. Awesome!
-Patti