I plan to thin with mineral spirits,and 69dartGT, yes I'll try that test you mentioned.

When I sprayed rustoleum through my HVLP gun, I thinned with mineral spirits, maybe 5-10% and it sprayed easy and layed down really well with the 1.5 or 1.5mm tip that came with the harbor freight gun. Plus it covered 95% with no runs. Where it didn't cover 100%, I just sprayed some more on top of it after a few minutes. Seemed to work well.
It was not a very thin mixture. I'll aim for similar with brightside which again should be 5-10%, maybe closer to 10% since brightside is thicker on its own than rustoleum and a little more thinner should promote self leveling.