Reserve capacity is time in minutes under a 25 amp load. So simply divide by 60, and multiply by 25 to get effective capacity in Ah. For big loads, this is more useful than Ah, as those tests are typically done at lower loads. As with all batteries, the greater the load, the lower the effective capacity (due to internal resistance and other effects). For example, a car battery will likely last 11x as long at 2.5A than at 25A, even though the load is only 1/10 as big. At 250A, it would likely last 1/15 or 1/20 as long than at 25A.
In general, to have good voltage margin, divide reserve capacity by 3 and you will have a reasonable (20-30%) margin on top of the resulting Ah rating. As some devices are voltage sensitive, and voltage declines as the battery depletes, this will result in a more consistent power source. Also, if you want to further reduce that by 20-25%, the life cycle of the battery will improve dramatically--typically, a 20-25% reduction in depth of discharge yields a 3-10x increase in cycle lifetime
