I always figure out the calories for my chicken breast when it is raw. I think it's more accurate that way. The chicken will weigh less after it is cooked, but the amount that the weight is reduced varies based on how long you cooked the chicken for and what temperature you cook it at. Cooking it longer will cause more water to cook out of it. So it's hard to know if your 3 oz of cooked chicken breast is the same amount of meat and calories as someone else's 3 oz of cooked chicken breast. But 4 oz of raw chicken breast (which usually ends up being 2 to 3 oz cooked) is the same no matter who is doing the weighing (unless the manufacturer has injected water into it, in which case, go with the calories indicated on the packaging, not on Fitday).
So, if I'm eating chicken breast, I weigh it while it is raw and log the calories for however many oz of raw chicken I have. Then I don't bother to weigh it again after I've cooked it. Some of the water cooks out but all the calories are still there.