I have used
7.5 mil mylar - https://amzn.to/2NtCR9X
magnet to secure the bed - https://amzn.to/2wVoz8N
settings
thickness - 0.01
speed - full
power - 50/55
focus - auto
To get more cleaner stencil, I would say 4 mil stencil should work more effectively with GF which i have ordered them here - https://amzn.to/2N0HXew
I will try and post it soon .
Happy Crafting 