If you Google "recipe builder," it's the first one that comes up. I'm hesitant to provide a link because it is a competitor site, and I'd rather not get on the bad side of the moderators
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Realistically, though, other than doing a search of "calories crustless pizza," and taking an average of what you find, I can't imagine where to get an average value.
I read an article a while ago and they did study the calorie counts for pizza, and they were notoriously inaccurate, so I think taking your best educated guess is one thing, but you could eventually reach a point of diminishing returns by which you spend so much time trying to figure it out that it drives you nuts.