I have feeler gauges, but I just end up using 0.88mm guitar picks most of the time.Just a fyi I just did the conversion as the gap on the mill is usually done in inches so, that said a .88mm guitar pick equates to a .35" gap which is it seems where most people are tuning to.
I'm using Dunlop Tortex (green) picks, BTW. If I need to adjust my gap I cinch down on a pick kind of tight, so I'm thinking I'm at about 0.034. I'll spot check my mill every few months if I think of it, or if I am trying to troubleshoot an efficiency issue. Otherwise I'm pretty much in the "set it and forget it" camp.
And I'm tuning to C# standard on my 6-string and either B-standard or drop-A on my 7