#2 and #3 foot lengths are defined below where J is the distance measured from the front side of the mast to the intersection of the deck and he
To clarify, the size is defined by the LP (luff perpendicular), not directly the foot size. The LP will be a function of the foot length and clew height (or leech length).
If you want to get close, you can just look at foot size, but the LP will measure smaller than the foot size.
For a deck-sweeper, they are close to 1:1. For a high clew working jib, the LP will be fairly less than the foot.
Final thought, the % shown in the table is a guideline, but not hard fixed. Some people prefer a 150 or 145% for a #1... some consider a #3 as 100 or 105%... so YMMV, but a good starting point.
I’m sure you guys know probably knew this, and were just simplifying, but posting for others that may search this thread in the future