Firstly, your bed plate shouldn’t matter to anything, as long as it’s level and brings your type/plates up to type high.
Vandercook recommends no more than .03 over the undercut–so you’d want you paper and tympan/packing thickness to total .43. I find that with Lettra (because it’s so soft), you want a bit more than that–closer to .50.
(I’m not sure what Lettra you’re using that’s 80lb. Is it the text weight?)
Lettra 110# is .021
Tympan is generally a standard .06
I don’t use mylar, so I can’t speak to its thickness, but you should be able to find that info on the packaging and/or the place you purchased it.
With 110# Lettra, I use 1 tympan top sheet (the diecut piece that goes under the gripper bar and around the tightening rod), plus 3-5 tympan packing sheets, depending on the amount of impression I want, and whether the printing is double sided.