The question was how to meter, not how to eyeball the histogram or guesstimate from guide numbers. I mean no offense to those who answered this way. Just that the original question had specifically to do with metering.

As far as I know, the most reliable way to meter a flash is to trigger it from an incident flash meter. You can then know exactly how much light is falling on your subject. Period. It takes diffraction/absorption of bounce surfaces and distance into account because all it is doing is measuring what falls on the subject.

I use a Sekonic, and I think it's probably overkill for what you're doing -- if you wanted to add an expensive flash meter you would probably be better off getting some ETTL flashes instead.

Getting back to my first comments, it is probably way more productive to do as @pat_farrell suggested -- shoot some test shots until you're happy with the result as it is reflected in the histogram. Then you have a good setup until some factor in your scene changes, such as subject position.

The important thing to remember about metering with digital cameras is that you can use the meter as a guide and tune to suit. You don't have to constantly take new readings. The same for test shots. Unless you alter things drastically, you can just move your exposure setting around -- just be aware of the histogram with each shot.