Interesting... my formula is slightly different and simpler I think:
(distance + slope) *x -> ex: 18ft+ 2i ->20ft *x
x is a conversion factor for the speed of the green - this
0.7-0.725 for championship - though have not played these in a long time I don't remember
0.75 -> tournament
0.8 -> v fast
0.9 -> fast
1 -> standard
1.1 -> slow
this gives the right distance to the hole - I then add 0.5-1 to that number to ensure I pass the hole and give some margin error - in that you get almost invariably <1ft from the hole - unless you mess up reading the green.
With longer putts (>30ft) with a steep downhill slope I find need to be conservative and remove 2-3 feet usually.
If slopes are sideways and steep that changes a little as ball has to travel uphill first. Strong wind also changes. It takes a bit of thinking but this formula made me improve my putting average massively.
the difficulty is reading the slope correctly, but at least the forse is sorted.
OBVIOUSLY putter pal is needed, so beginners might not really understand how this degree of precision is possible.