I have been doing my own visual alignments for some years, I use a No 0 dental mirror ($4) and a single led Book light ($2), the second thing I do on a secondhand horn after a good cleaning.
2 examples come to mind, a Bach 1964 Mt Vernon Large bore purchased from a local shop on consignment from the estate of the original owner, it had been serviced by the shop, cleaned and new felts fitted.On getting it home I did not feel it lived up to the hype the Mt Vernons receive. Checking the valve alignment all valves werre out in the up position, rectifing this made a great player, all my friends that have tried it want it.
Purchased a 1941 York Custom from an on line dealer I had several transactions with, it arrived with a note saying "I hope you are happy with it, it does not play very well", this was an understatement, there was no note center at all! The valves were a long way out of alignment, rectifyng made it playable but I was not happy with it, the notes now centered but the tone was dull and lifeless, it had at one stage had the leadpipe bent and straightened not very well, the rear brace resoldered with an excess of solder, on heating up to remove the excess solder the brace let go with a loud pop as the tension was released, plays much better now.
One of these days I will dismantle the whole horn and reassemble stress free.
Valve alignment in most cases is a compromise, I have measured the difference in the spacing of the input and output ports of valves and found only 4 trumpets that I feel are within good engineering tolerance 0.002", coincidently they are my best players, 1949 Olds Super Recording Eclipse and Taylor with Bauerfine valves and 2020 Jerome Wiss 6/20.
Regards, Stuart.