I usually do what vabeachgirlNYC does, but if you can't, maybe you can go by fractions. If you know the slice of round pizza has, say 300 calories, then the whole pizza would have 2400 if it has 8 slices. So if your pizza is the equivalent size and has 2400 calories, then just divide 2400 by the number of squares and you should have your cals per square.
The other thing you could do would be to weigh it. I don't know if FitDay does or not, but many calorie counting sites will give the caloric values in grams.
And now that you don't have to be perfect, you can be good.