i wouldn't bother with the trunk as you were planniig. really if there is no pressure back there then there is no point. THOUGH, i would take care of the wheel wells(not the spare tire well) just to help reduce some road noise.
80sqft will deff not be enough.
i picked up 80sqft and did my inner front door walls and the outer door wall with multiple on the outer door wall, 2-3 layers on my rear deck and rear sails, and about 30% of my trunk working my way from the rear seats towards my trunk opening. the deadening helped some in the trunk but not much. it's not easy to deaden a civic with a 15"btl on 3k(at that time). it did reduce a lot of the road noise from the rear though because i did focus on the wheel wells knowing that the trunk wasn't going to get completed.
overall i'm going to leave the trunk as is. i sold the BTL and picked up an lms5400 and will be forward firing it while aero porting it through the rear decks speaker holes. i plan on sealing it off as much as i can so none to very minimal amount of pressure is making it to the trunk.
i just picked up 20ft of edead 80 so i can add some deadening to the areas that brace around the rear seat to trunk opening. i'll multi layer it then with the rest i'll throw it on the floor in the front wheel well firewall section to try to reduce a little more road noise. i m not baller enough to buy a small amount of deadener for an arm and a leg to only cover so little. yes it may be top brand but to me i really don't care. i'd have to win the lotto to buy some SS or such.