This is based on our experience handbuilding multi-layered molds for this size of object. We found that if we used the full volume of space that the objects occupy, we had about twice the investment that we needed.
The multiplier of .64 for the water weight in metric units works for investments mixed at a ratio of 1 water:1.75 investment as well as investments mixed at 1 water:2 investment. In the latter formula you will have more total material. The .64 multiplier originated in technical materials provided by US Gypsum (USG). They are the manufacturer of the plaster used in the investment.
If you want to do this same thing using imperial measurements (cubic inches), use a multiplier of .023 for the water weight in pounds.
I hope that this is helpful.