The higher the fingers, the more leverage felt at the clutch lever. Too high of fingers they could rub on the case. Originally you could buy different thickness of steel plates. All good things go to sh-t so in todays world you have to sand the friction plates. (Or burn the clutch wildly against a wall) The 17.5 is straight down from the highest point of the fingers. That is what matters most! Sure you can measure the total thickness, but that what really matters is when it is all tightened up.