It's an empirical number that may not be accurate. But the theory is for a consistent 10-15 yard gap through the set. If you hit a P 120 yards, then you need an 8% gain with a 9 iron for 130 yards. If you hit a 5 iron 170 yards, then a 4 iron to go 180 yards only needs to gain 6% (10 yards/170 yards). So three quarters of 4 degrees (3 degrees) should cover it.

Depending on your club head speed, you may not get 10-15 yards from a 5 iron to a 4 iron with only 3 degrees. I don't think even 5 degrees from a 5 to a 4 iron will work for me. Hence the theory breaks down and I need to look at a hybrid and/or higher loft fairway wood for proper gapping.

Great explanation, Don. Very well articulated.