It depends how you work, I have a 10mm shank edge finder as my primary reference tool in the tool table, but I also have a modified electronic LED edge finder with a sprung ball tip which is nice.
Copier paper is reliably 0.1mm thick for edge and surface finding with actual tooling, if you want to use a small hole as a reference get a pointed centre finder, lower it into the hole until the taper section encounters friction then feel the shaft (f'nar, f'nar) for the step and jog to position, like the Mk1 Eyeball the Mk1 Finger is a much underestimated tool.