Up till now, I've tended to scale my first layer according to the print quality, so a .12 first layer for a 0.08mm print, and 0.28mm for a 0.2mm print.
After changing to a PLA which isn't sticking well, I'm wondering if the first layer is best determined by the printer/tolerance/material, rather than the overall print quality settings. Am I going to get more predictable results if I stick to a 0.12mm first layer regardless? This is with a 0.4mm nozzle on an Anet-A8.
I find that a thicker first layer helps with adhesion. Since the first layer gets a little squished into the build plate, it tends to be slightly thinner than set in the slicer. When the first lay is too thin, I have had problems with the very thin layer peeling during printing.
That being said, .12 is actually pretty high compared the first layers I have managed to get to work on my A8. At that thickness, it is likely the PLA and not the first layer thickness that is giving you issues.