Popa, with your permission, if I can add to this thread another question that has been bothering me on this subject.
However, what’s bothering me is that as the Gemara concludes, the pool consisted of 3 amos of a square tub, followed by 2 amos of a round tub. If my calculations are correct, the ratio of height-to-width is larger in the case of a round tub than a square tub. So the premise Rashi sets forth on the height/width ratio, is this true for round tubs or for square tubs? And in this case we have a combination of the two.