I go with the 1 1/2 hours per pound + 1 lb so your 7.7 becomes 8.7 x 1.5 = 13 hours, but why not add either a second or something else?
This will double your fuel efficiency, if you do a second it wont take double the time, and you can cool, chill and freeze the second for a pre-cooked meal next week, or put a joint of beef on the bottom rack and ignore it till the pork is cooked, or maybe throw on a couple of racks of ribs.
Any thing that you like can be added without a big time penalty, if you are cooking only for two or three, (and this is the only reason you are thinking of just doing the one item), ask your neighbors or friends if they are interested and want to have you smoke something for them, if you make it the same item stick a toothpick (or a skewer or cut/trim) in yours to diferentiate between them.
You could cook 4 butts in about 15 hours, so for 2 extra hours you can please yourselves and 2 or 3 others.
Think about asking around if you can't fill it for yourselves, you may be pleasantly surprised at the outcome!