Here are the tools I use. The first is a beading tool that Henry Taylor makes. Its 3/8" across. On this beading tool goes a Sorby sizing tool. With proper use it will give you repeatable turned diameters without using a separate caliper.
The other tool is a dial caliper, and it is a real must in my book. You'll want to measure the tenon as you test it and adjust your beading/sizing tool to tweak the fit until everything is just right. It can be just a few thousandths that make a difference in a perfect fit so this tool will save you lots of time.
Using these tools allows me to adjust my fit for each kind of wood I use. Also, since different styles of drill bits leave a slightly different size hole, despite being the same diameter, this will allow me to adjust my fit for each unique set up and drilling bit I use.
Hope this gives you a little extra help in the shop for that next project!
By the way, these tools can be found at Lee Valley.