I posted this as a response to a question on another thread, but thought I should post it as a separate thread. Perhaps that would make it easier to find when searching.
The attached spreadsheet is based on the classic cat curve formula of
Y = a * cosh (x/a) but some simplifications have been made that match the conditions where we use cat curves in tarps.
The assumptions are that the cat curve is symmetrical and the simplification is that a unit curve is used. In other words the distance from the center of the curve to the end is 1. It really doesn't matter what the unit is; feet, inches, etc. An offset is calculated based on a proportion of length. This means it can work on any curve given just the length and amount of cut.
After simplification, the formula transforms to y = cosh(x) - cosh(1)
Here is the attached spreadsheet that includes simple instructions.