The color you decide on makes a difference. Darker colors cover with fewer coats.
I've done color changes on almost every car that I have painted. The A body cars usually take about 3/4 of a gallon of what you refer to as "Sprayable" to cover the engine bay, door jambs and trunk. I've yet to repaint the undersides though. I've sprayed red, black, blue and white. Black covered in 2 coats. Red took 3. My next car will be Detonator Yellow. Its a very "tangy" shade. I'd resurrecting a derelict 74 Duster 360 and the factory GY3 was just too mild for me.