Found my own answer perhaps
Sopwith: (c + 0.2)/(c-1)
Bergstraesser: (c + 0.5)/(c-0.75)
Bergstrasser might be a little more accurate at low spring indexes. With spring indexes above 3, all should are similar enough for my needs
I have always used the Wahl Stress correction factor. There are at least two others: Sopwith and Bergstraesser. Does anyone have the formulas for these? What are the differences, strengths, weakness, etc?
