How I have dealt with this question in the past: When I finished my last brew, a pale ale, I filled two 5 ltr. kegs, which were each primed with about a teaspoon worth of corn sugar, then I looked at the level of remaining beer, and used an online priming guide. It asked for quantity of beer, temprature, and desired volumes of co2 (dependant on what style of beer you are bottling). Then I measured out the correct number of ounces with a postal scale, added it to a small bit of boiling water, then mixed it in with the beer in the bottling bucket and bottled it. Seemed to work well.
Here is the link to the priming guide: