Along with the real risk of under-thinking, there's also the risk of over-thinking I reckon.
A certified test bar is good, very good - but a decent chunk of silver steel can act as a pretty reasonable guide, and unless one is doing long tapers, or needing to work - guaranteed - to a fraction of a thou, you'll be OK without going to the nth degree.
Or that's what I have found, for the sorts of things most of us do day to day.
Worst thing in the world is not to dare do something because one is 'frit'.
I'd get that headstock and tailstock lined up best you can, Richard, then play for a bit. Without panicking if small errors continue to exist. Tooling, play in the cross slide and topslide, operator over-eagerness re depths of cut, speeds, tool-centring etc . . . . a million things can cause small inaccuracies, but 90% of them don't matter a whole lot, and may well not show up on 'short' workpieces.
Mistakes are the best catalyst for improvement, believe me!
Decent 'suds', as in - especially - a good quality cutting oil for steel, also contribute greatly in my 'umble. I use a not-to-be-diluted oil which is supposed to be good for tough work. It seems to be.
I am completely untrained, but can get short workpieces pretty good on my small Myford, to comfortably under half a thou if I want.
As an example (to encourage, not boast) . . .
I wanted a 'true' measurement of the wear on the T/S bush on my A some time back.
My measuring sticks are just the usual amateur ones and I didn't trust them or me for doing inside measurements of the bush. So I made a mandrel from decent mild steel bar to fit the bush spot-on and then measured it. When done I reckoned the bush was still within the 1.5 thou spec for 'good to go'
I sent the crank to my favourite UK shop (T&L Engineering in Bedforshire), with the cases and bush in situ, as there was other work to do. Which they did. But they sent the bush and T/S main journal back untouched, saying "1.5 thou +/-, very good, nothing to see there, carry on".
I was pleased and it was good for the confidence.
If it's true that the harder one works the luckier one gets in life, it's also true for the results obtained in sheds.