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You just need the diameter of the bottle and divide it by the diameter of the tower. This leads to the scalefactor. At best I think is, measuring the bottle filled, but not under pressure and substract twice the material thickness of the bottle, after you cut it open.
I don't have any bottles on hand, and I'm not sure if we have any standard available, looks like every bottle is different. Finding a straight one is the first goal, then measure it. You can use a measuring tape to get the circumference and divide this by pi to get the diameter.