<p>Up till now, I've tended to scale my first layer according to the print quality, so a 0.12&nbsp;mm first layer for a 0.08&nbsp;mm print, and 0.28&nbsp;mm for a 0.2&nbsp;mm print.</p>

<p>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 0.12&nbsp;mm first layer regardless? This is with a 0.4&nbsp;mm nozzle on an Anet-A8.</p>

Default settings for first layer height in Slic3r Prusa Edition print profiles regardless layer height is 0.2 mm.
If you need to improve bed adhesion then try tips from this video
3D Prints not sticking anymore? Watch this! 3DP101
by Maker's Muse. It's about using glue stick and spreading it using paper towel and isopropyl alcohol.
There are other possibilities how to improve bed adhesion, e.g.
Ultem sheet
or other printing surface like
BuildTak
.