Just been re-reading Alex Warlordy's book on Holleys "Super Tuning Holley Carburetors".
He quotes Jim Patrick of Hi Po Shop's method for sizing power-valves: get vacuum-reading at idle (in drive for auto), divide reading in half and add .5 inches. ie: 16 in of vacuum divided in half + .5 = 8.5 power-valve. That's about what Pat advises.
However, another statement has me confused: "You gain more economy from a higher number powervalve, and go richer earlier with a lower number valve". To me, that seems back to front, or am I missing something??