Hmmm I actually normally do the opposite (I use TheDailyPlate). I'll input all of the ingredients as a "meal" and then tell it the amount of that "meal" I ate.
So the pan of lasagna for me would be (1) serving and I would then just estimate the amount of the pan that I ate and input that as a fraction. So I ate one row of the pan and there were about 4 equal rows so I would input 0.25 and viola! I have all my counts.
I find this to be simpler and nets out to the same result.
I don't think it really matters, just whatever is easiest for you. If you wanted to hit a specific target # of calories in your meal it is tougher. You would have to add up all the ingredients, then divide by the number of calories you are targeting and cut a piece that small. I personally find that way to obsessive (are you really going to cut your lasagna into exactly 17 100 calorie pieces or whatever