So, you're wondering if there is a math equation for curls. Something like (Wet curly hair length is equal to "X" dry curly hair length).
There is no mathematical formula to explain the change in length between wet and dry curls. It's different for each person, and it depends on three factors: 1.) the porosity of your hair, 2.) the elasticity of your hair, and 3.) It depends on your curl type. An example would be if your curly hair is really, really, really tight and coily, it’s very porous and also its elasticity is nice and strong - meaning that it’s not damaged by heat or chemicals. Then yes the shrinkage can be a lot. Again, it's different for each person, and again it depends on those three factors.