OK then, you have a battery with fake capacity even if it was excellent. So 70Ah is really 35Ah. You say 12.7 volts is good, but have seen 13.4 volts.
So lets give said battery a little chance and call the volts 12.5 So that would give you 347.5 Watts/hours.
A 21 watt 12volt bulb draws 1.68...