What would be the allowable slope for hole under Jeff's formula at a green speed of around 8 on the stimp?
I would be interested to see how more boldly sloped, slower greens would play.
Old Shell Wonderful World of Golf episodes give some clue but I am frequently surprised at how soft the greens are in those episodes.
Jerry Lemons, ASGCA did a big study a few years back, and you can access that chart on line. For 8 stimp, his maximum (but marginal cup slope was actually about 2.65 deg., or 5.86% slope. A degree is about 45% of a slope. A % slope is about 2.2X a degree.
I was once called to a project by contractor who thought out green grades were all out of whack, but had his digital level set to degrees and not % slope.
The good news about Jerry's chart, is that greens can actually be a bit steeper than the USGA oft cited 2-3%.
As to how much areas would be lost under Jason's question, I would guess none, as TD suggests. The green interiors might just be steeper overall in the old days.
TD doesn't define "old courses" but generally, I have seen 1950's greens at 6% through cupping areas, down to 4% in the 60's, 3% in the 70's, 2.5% in the 80's, 2% in the 90's, and since 2000, most architects seem to have stayed at 2%, but some have made the majority of their interior green surfaces below that, from 1.5-1.8% All in general of course.
Tom mentions the difficulty of converting degrees to percent. I recall talking to Pete Dye about the steepest slopes he designed for cup spaces, and he used "a quarter inch per foot." It took some math to figure it was about 2.25% when he was designing for tournament venues, which I presume he almost always did.