Bit hard to judge the size of the briquettes you use.
I run 2 side by side leaning on the ones in front with a row of one on top touching, with wood/pods as required and get about 12 hours at 225 to 250 using the bottom vent to control.
ambient would be about 65.
Got the idea from Kevin at the ABBQ forum
Aussie Aussie Aussie