I believe Pillsy's calculations are correct.
The club only moves 19 times, but the last movement ends at 95%. You have to add 1 for the last 5%, making 20. Like Pillsy I use the %.
So, if you start at 100% with a 200 yard club and go down 1, you're at 95%(190 yds); down 2 - 90%(180 yds) and so on... The yardage isn't going to be exact, but it's pretty close depending on the lie, wind conditions, elevation to target and amount of spin you use.
GL