… how do I know when its “right”?

I’ve three (all next to each other) to replace at the weekend involving tubeless & tape removal / replacement and I’d rather not do it again if possible…..

As tight as possible.

As tight as possible.

unless they’re Stans…

I use one of those PArk Tension meters and generally aim for the middle tension on the range for the spoke type/diameter I’m using.

I should add, without a tensioner….. aka by hand.

I guess you need to get a feel for how much you need to tighten before the nipples round off – that’s the effective limit.

Hard to describe…squeezing the DS pretty hard, you shouldn’t get much flex at all.

got a guitar tuner or perfect pitch ? though my spokes twang is very flat

If you’ve got access to a known good wheel, pluck a few spokes with your finger nail like a guitar string and listen to the noise, it should be quite a high, bright “ping”. Aim for something similar, it doens’t need to be exactly the same, and probably won’t be, but just aim for somehing similar, not a flat twanging noise.
I’ve seen an article in the past that demonstrated that even an average person that doesn’t have perfect pitch can easily hear a difference in tension between spokes about 4 times smaller than you can reliably measure with a tension meter.
Edit:
If you are only doing a few spokes, they should match the spokes they are next to, or as close as they can be and get the wheel straight. The tone will be different on the two sides of the wheel due to dishing but all spokes on each side would ideally sound the same when plucked.