Good answer Bill. Mostly, the foam is just more dead weight. It doesn't become effective until you are knee deep in water. It is much easier to get your boat towed back to safety when it only has a couple inches of free-board than to salvage it off of the bottom. I plan to have approx. 6 cu ft of foam to counteract all the parts of the boat that have a negative buoyancy such as the batteries, motor etc...
Even though I feel my boat will be practically unsinkable... remember the Titanic.
Here is a picture of the foam that will be below the sole. (floorboards) The foam does not flow nice and level but rises a lot like dough in a bread pan.
In these four sections I made 3 smaller pours instead of one big one and will have much less waste.

- Foam poured in 4 sections under the sole
I have used 1 gallon of foam. Even though 1 gallon should make 5 cu. ft. of foam, I estimate that I will end up with 3-3/4 cu.ft. because of the waste I have had. I am sure 1 more gallon will be more than enough to complete my flotation. It would be much easier if I had deeper spaces to fill.