what lens for rangefinder adjustment Leica m

Hi guys, Im going to adjust the vertical alignment on my Leica m6,
At the moment the longest lens a have is a 35mm ltm, adopted with a cheap adopter of
''questionable quality''
not ideal for fine tuning I imagine, I hear people referring to using a 50mm lens when adjusting rangefinders, I don't see why a 40mm wouldn't work but if I'm missing something pleas let me know,
also if ultimate accuracy is important would a longer lens like a 90 or 135mm be better?.

For vertical alignment it doesn't matter a bit. For horizontal, you want to adjust the camera to your fussiest lens and let the rest take care of themselves. That's the longest, fastest one you have, usually. If that's your 35mm, then that's the one.