My mistake I was calculating with 5w ones earlier , that's how I arrived at the 0.42. Using 8w would actually be 0.66 amps per bulb (3.3 amps total).
The scenario I am calculating this for is while camping with family. We normally run lights in the evening with propane tanks/lanterns. This is costly to refill, a pain to bring the tanks home/back etc.
What I was proposing was adding fixtures that would be permanent at the camp site and just add the deep cycle battery/lights once we get there.
Normally we are there for 2 nights, operating lanterns from 7pm until 2am maximum, so about 7 hours of use per night.
Using 3.3 amps on an average night would drain about 27% of the battery. I arrived at that number by 85 ah / 3.3 amps drawn = 25.76 hours total life. 7 hours is 27% of 25.76 hours.