question about baking between tiles

Hi, all. I have a question about baking between tiles. I want some
perfectly flat pieces, and I thought the best way to avoid an possible
distortion would be to bake them between tiles lined with parchment.
The clay will be about 1/8 inch thick. How long do y'all think they
should bake? Should I perhaps preheat the top tile before placing it on
the clay? I imagine the time will be longer, but I'm just not sure what
adjustment to make, and I have to be careful because the colors will be
pastel and I don't want any darkening. I'll be using Premo for this
project. Any advice would be appreciated!

--
Anne Onimos
http://losingsanity.blogspot.com

Figure out what to remove if you want to email me ;-)