I contacted my local agent at Deka Batteries for information when I was programming my battery monitor:
The agent for Bucks county is :
Phone : 610-944-3140 ext. 3800
Email : email@example.com
EAST PENN MANUFACTURING COMPANY INC
LYON STATION, PA 19536
I also have a pair of Deka DP-24's
He said my battery is 75 AH and that seems to work OK as a setting in the computer; but is in reality "high" for this Dual Purpose Marine battery. I have been waiting for them to die - or even get sick - (5 years on the pair now) but they seem to want to live forever.
To convert RC (Minutes of Reserve Capacity at 25 AMPS - the "Industry Standard") to AH (Amp Hours - hours capacity at 20 amp draw) the minutes available at 25 amps is multiplied by 0.4167.
Trying to use the 110 minutes at 23 amps (stamped on the battery) is tricky due to the Peukert Effect which makes AH capacity non-linear based on actual demand. (See chart and document on the effect.)
Using Awwall's 120 minutes at 25 amp RC; the calculation results in a 50 AH capacity as he states and seems much more correct than the 75 AH quoted by the Deka Rep back in 2009 when the batteries were new.
My performance is most likely even worse when you conciser the charge/discharge cycles my battery has undergone in 5 years.
See capacity reduction vs charge cycles graph as well.