Of course the best sure way is to use the hydrometer method. I however use this chart to get me in the ballpark of the battery state of charge when I am camping off the power grid. It of course is approximate but served me well over the past few years of re-charging my batteries when they hit 12.0VDC back up to at least 90% SOC. I can do around 12-14 daily runs of these 50% to 90% charge states and them I must do a full 100% state of charge to keep from doing damage to the batteries...
My 255AH capacity of Batteries are just now dropping off in performance since installed in 2008.
