Always a tricky one Tim 😕
I’ll be honest and say I usually tack stuff on with magnetics and stand back to see how it looks. Usually the heaviest or strongest line gives an indication of looking ‘right’ and then I use that as a datum for everything else.
Occasionally you need to fudge it a little so text on doors (for example) doesn’t look too far away from being ‘straight’.
Ideally of course there is a strong datum line (black bump strips are great) If there isn’t then sill line or door bottom gives a reasonable strating place to measure from if you’re not confident about your ‘eye’.
The new Transits are good fun . . .
Cheers
Paul