Understanding Amp Hours

[camaraderie]

Actually it is about 2.12V per cell which works out to 12.72...so 12.7-12.8 is the right number for "full" depending on the accuracy of your meter & temp. 12.1 is about right for a 50% discharge...I try to aim for 12.25 to give myself a little leeway since falling below 50% regularly costs money...but you are correct.
For the benefit of others...Xantrex/Heart gives the following #'s for discharge state.
% VOLTS
100 12.7
90 12.8
80 12.7
70 12.6
80 12.5
70 12.3
60 12.2
50 12.1
40 12.0
30 11.8
20 11.7
10 11.6
0 <=11.6

It seems that lots of folks differ a little on what 100% and 50% is so I guess as a practical matter since you never are gonna turn everything off for long on a cruising boat...staying with a little bit higher rating in Voltage is not a bad thing since the bank will never fully "settle".