re: why not tighter cal of Mylar film.
Mylar (aka stabilized polyester, Estar) changes size a lot with humidity (around 8 mils in 10 inches for every 10% RH change, and less with temperature (around 1 mil in 10 inches for every 10 degrees F change).
A sheet can also be distorted several thousandths (inch) by the holding method. Some (not properly stabilized) sheets can change size differently in different directions.
GR