John Palmer says in How to Brew: Everything You Need To Know To Brew Beer Right The First Time that if you're priming per bottle with corn sugar you should use a teaspoon of corn sugar per bottle. I figure that the beers are at least partially carbonated, so I want to use less than a teaspoon.
After consulting with Melanie (the house scientist), I decided to go with five trials at three different concentrations: 1/2 tsp, 1/4 tsp, and 1/8 tsp. Unfortunately, the sugar gives the existing carbonation something to bind to, which means as soon as I add the sugar it foams over. Ultimately I had to have Melanie add the sugar while I stood over the bottle with the capper ready to go.
Game plan now: give them about a week to carb up, then stick them in the fridge for a few days and test them...at least whichever ones don't blow up on me.