While I have experience with both those materials (and will be able to help in the future), I'd struggle to estimate it off the top of my head.
Dan99 casted a Mark VI helmet last year I believe and may be able to give you an idea on volume.
You'll also want to pick up a small bottle of Smooth On's Thi-vex, which is used to thicken up the outer layers of silicone. Also research what material you'll want to use for a jacket mold. If you need any resources on mold making and slush casting, just give a shout and and I'll be able to link some.