"I get about " Bear in mind that unless you are using a trivial load, like a ten watt light bulb, whatever you have connected will probably be sensitive to the loss of battery voltage as the amps are being drawn out of the battery.
Fresh new batteries are typically 12.8-12.6 volts, so using "12" volts is also a bit deceptive. A "12" volt battery starting at 12.6-12.7 volts (typical) will basically be dead at a point one volt lower. As the voltage drops, some loads won't care, others will grow dimmer (tungsten bulbs) and some, like electronics
, may begin to misbehave or stop working.
So "watt hours" can be accurate--but still not tell you how much useable power you will get for any specific purpose. As a ballpark estimate, it still is perfectly good.