Well, to clarify a little, the boilover is the foam that begins to form on the surface of the wort as it starts reaching boiling temperatures, you know, that creamy stuff that you have all dipped your finger in to see how it tastes.
Thats what is mostly spilling out of the pot, running down the sides of the pot and then baking onto the side of the kettle.
As far as how much liquid it is, not all that much, what spills out of the kettle is mostly the foam and a small amount of liquid, I'd estimate it at around 1-2 pints of liquid.
My thinking is that all that foamy stuff is probably at least in some part extracted malt sugars? But its probably not enough to bring the gravity down by 10 points though now that I really think about it.
And a few specifics about the grainbill, its an AG recipe with a 5.5gal yield and it is 20lbs of grain.
Yes 20. Mash is at 158* for 90 mins. 90 minute boil. 8 gallons boiled down to 5.5 of course.