I have a 1987 Pacific Seacraft 34, hull #67, and have measured the fuel remaining at each level on the fuel gauge. The tank is original and the sticker says manufacturer is Vic Berry Sheet Metal, 760 Newton Way, Costa Mesa, CA, tank capacity 30 gals.
I am not saying that my measurements apply to your tank, but if your tank is from the same manufacturer they might, so I give them below.
First, with the inspection port on the tank open I found that the tank was not completely full when the fuel gauge read F. It is not actually completely full until the gauge pointer just touches the upper-left corner of the black square on the right side, which is above the F mark.
The amount of fuel remaining in the tank at different levels of the fuel gauge I measured approximately as follows:
-gauge pointer just touches the upper-left corner of the black square on the right side, 30 gal
-gauge pointer at F, 23 gal
-gauge pointer at 3/4, 15 gal
-gauge pointer at 1/2, 9 gal
S/V Ubiquity, Pacific Seacraft 34
My Pacific Seacraft 34 page on design, outfitting, and sailing characteristics