Allowing for material shrinkage

How can I use a finished part on a mold assembly and allow for matrial shrinkage for my final mold drawing? I have inserted my finished part onto the assembly by combining the part. I now have to allow for shrinkage in some areas as it is a silicone mold. So in essense I need to remove material in the cavity in a few of the areas. I needd to do this without altering the finished part drawing.

SolidworksPlastics And Mold Design