I too had conversations with Bill when I was starting "PierHead".
His general advice was to make the plastic masters 5% bigger than you wanted the final casting to be.
When I found my brilliant mould maker and casting master (Aidan Campbell), we experimented with two sizes of St Trillo plastic master. Bill was spot on, until...the alloy used was not always the same, so the properties varied. The differences were tiny, with a "Lady of Mann" varying by about 3mm in length. Not noticeable - unless you know to look for it.