I had heard that one of the issues was that the Germans preferred to do the fission calculations in the context of a plane rather than a sphere because the math

I had heard that one of the issues was that the Germans preferred to do the fission calculations in the context of a plane rather than a sphere because the math was easier. I read that at one point, but I have absolutely no idea where it came from. Maybe that was why they overestimated the required amount of fissile material?