I'd cram in as much as you can, cuz if it sinks you'll probably be paying for recovery of not only the boat, but also the petroleum damage to the environment. Of course if you drown, it'll be someone else's problem. I don't know the size of your boat, but I managed to get about 35 cu ft of foam into mine, which I think will at least keep her at the surface. Under the floor is the most common place for flotation, but getting some into the upper parts such as under the gunnels will help keep her upright. Do a calculation of how much your boat weighs in water and put in at least that much equivalent cubic measure of the weight of water. If you get almost enough to float it, you get to watch it sink more slowly, so I'd over-engineer if possible. There's some details on how I did mine in the thread linked below.