This is a subset of the density calculation.
Select the clay you use.
Select the silicone you use.
Enter the weight of the clay.
The resulting number is the mass of the silicone you need to replace the clay.
The densities used in these drop-downs are collected from TDS where I could find them.
The density of the materials marked with a star (*) are uncertain since I found these in MSDS or on the back of the container. I think they are correct, but no guarantee.