There is a volume calculator on makelure.com and alumilite.com sites ... here is the one on makelure's site ...
http://makelure.com/CalcVolume.cfmYour height seems a little high because you only need to cover your wigglie by 3/8" of silicone but perhaps you are talking about doing both sides. Basically the volume calculators will help take the dimensions of your mold box and subtract for you the dimensions of your original to calculate the amount of rubber you will need for your piece and the amount of resin you will need for your cast piece.
Put all your dimensions in and that will help you know a close approximation of your material needs.
Mike