They are definitely undercarbed. Co2 dissolves into colder liquids more readily, so the co2 that's in the headspace in warmer temperatures goes into the beer. That's what you want of course, but you'll need to use more priming sugar.
I don't do it by the bottle, I use 4 ounces (by weight) of priming sugar per 5 gallons. You would probably get better, and more reliable results that way.
But if you want to do it by the bottle, I'd have to think it'd be closer to a teaspoon per bottle. We'll have to wait for someone to come along here to tell us how much for sure. I'm sure others do that.