Hi Rich.
Thanks for the heads up re modelmaster. Is this the type product you refer to? https://modelmaster.uk/4mm-locomotive-lining-packs/17713-mm4000b-black-lining-05mm-wide.html If its a waterslide transfer and only 0.5mm thick, I don't see how one can possibly get it absolutely 100% dead straight, especially over a paint surface join between the white and blue paint. BTW, when I used the term 'dark art', I was of course referring to the use of a lining pen nib with wet paint, which from looking at many YouTubes seems a very specialised skill.
Check out lining:
(it should start 2h21m16s in to the very long video)
Cheers
Noel
PS: The guys in this training video are brilliant but its 3hrs long so recommend only watching the segments of interest.