Priming sugar in kegs

what would be the proper amount of priming sugar for a 5 gallon batch of chocolate stout and a 5 gallon batch of a super hoppy APA, if i was to prime in my corny keg instead of force carbonating?

This will help you out.

http://hbd.org/cgi-bin/recipator/recipator/carbonation.html

We use half the amount that is used for bottling. We use 5 ounces for bottles and 2.5 ounces for kegging. Boiled up in water of course.
BTW, I really don't know why you use half the amount. It said that in the kegging instructions and that's what we've always done.

+1 on putting about 1/2 of what you would normally put into your bottles, the one additional step I take is to hook the keg up to the co2 system and put 15 psi of pressure in the keg to make sure the keg seals up tight. On a few occasions I have keg carbonated and had problems because the lid didn't seal correctly because there was no pressure to hold it in place.

I think using less sugar anticipates that you put a little CO2 in the keg to get a good seal, and accounts for there being less headspace when compared with bottles. But I am not 100% sure of that.