IMO Greg's got the easiest solution and is more or less what I use.
My method(?) goes like this:
With a 150mm/6" lens,
275mm/9"=1 stop,
300mm/12"=2 stops.
Halfway between gets half the correction 1/3 gets 1/3 the correction. Plus or minus depending which side you're measuring from.
With my hand, 9" is the spread from thumb to little finger whilst hanging ten.