I've got a little tach -- not a pro model, but reliable enough. I've tried setting them to spec for idle where I can, but in some cases (and it seems to be on well broken-in saws), the spec is too slow and it idles a little more reliably if you run it a bit faster than that.
Partner had a blurb in their manual for a P5000 that basically said to set the saw to idle below engagement and as low as you can where it still idles (like you said), but then test it by turning the saw on its side and back upright quickly (maybe a few times). If changing the orientation of the saw quickly brings it close to stall then its too low.
If the spec is too slow to idle reliably, or I don't have the spec, I use Partner's test. Ideally that's just enough below clutch engagement that you don't have to fine tune every time the weather changes. ( I think that guys who know their saws and have been using them for years have a pretty good sense of where that is without so much thinking -- for me, I find the Partner test helpful.)