To figure out the displacement of 45s, just measure the full square or the area to be displaced and divide it in half, so say you are working with a corner that is 4"x4" by 18" tall.
4 x 4 x 18 = 288/1728 = .17 cubic feet. Divide that in half for the total displacement.
.17/2 = .08 cubic feet
The red line represents where the 45 will be. the black box around it represents the "square" measurement. I find this easier to do than the volume of a pyramid.
I can't figure out sketchup either, lol. As you can see I prefer Paint. :P