the amp/hr rating given by the manufacture almost always is based on full charge
and then discharging it to something like 1.75volts per cell
which is basically fully depleting the poor thing.
this is actually not dishonest, because there is a huge industry that needs batteries to do just that a half dozen times or so over the lifespan of the battery. things like telecom's and even the little ups under your puter desk use this theory of operation.
about the only way you are going to know for sure, is hook up the battery and start using it, time how long it can carry your load till it hits something like 11.5 volts, and then make appropriate sizing changes.
this would likely give you good service over a fairly long life
me i would quit at 12.2 or so, but i am talking hundreds of amp/hrs in capacity.
bob g
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