So far for my silicone rubber molds using Smooth-On OOMOO 30 I have been determing how much I need by weight.
My post Arisu's Head Silicone Mold: Part 1 goes indepth about how I determined the amount of OOMOO 30 I needed in ounces by first determining cubic inches of space to fill. Even with a scale I sometimes prefer to measure by volume instead of by weight. After I made a few molds I was able to make myself a conversion table. It is not 100% accurate, but it gives a rough idea.
Part A, Part B, and Total are all measured in ounces. I added horizontal highlights to help me reference numbers better. I can clearly see that if I want to make 11 oz of silicone rubber, that I should mixing 1 cup of rubber. That means 1/2 cup of A and 1/2 cup of B. That 1/2 cup of A should weight close to 4.8 oz and 1/2 cup of B should weigh close to 6.24 oz.
Ratio by Weight: 100A:130B
Ratio by Volume: 1A:1B
NOTE: Your results may vary, but this table is working out well for me.