Dennis,
Thanks for your input. Both you and Al W. mentioned changes in the size of the rubber mold over the course of its use. THAT is really a surprise. Your last sentence - "If you can't quantify it, you can't allow for it, so people build masters full size and live with the result." - may well sum up the situation in which I would find myself.

In this case and that of 3-D printing, why can't things be simple? VBG
Gene Green