TubeNet

Been considering having a dependent 5th valve added to the 4th valve tubing.
What is the common way to tune it?
I don't know if this makes any sense but:
Should one think of it as a long whole step replacing the 1 valve (like the independent setup) or just as engaging a minor lengthening of the 4th valve tubing using normal fingering?

Dependent 5th valves are disfavored because they can only work with a minimal number of valve combinations, and all of them including, of course, the 4th valve. Yes, as a long whole step, that will get (on a CC tuba) your low F a few ledger lines down in tune, and E below that in tune with a little lipping, but that is all. By making it dependent, instead of independent, among the alternate fingerings you lose are whole step trills or other technical passages that can be fingered 1+T and only having to move one valve, rather than play the passage 2+3 to 1, which may cause burbles even for the most technically proficient players, while you still get the advantages of the long whole step plus 4th to get the low F in tune as well as alternate fingerings where required for other notes throughout the scale.

These and other reasons are why conventional CC 5-valve tubas are configured as they are. Please reconsider not installing a dependent 5th valve.

I have two dependant valves in my 4th valve circuit. Flat whole tone and flat half tone, on a Bb tuba I play in brass band. If I only could have one it would be the flat half tone, to cover 4 + 2 combinations. Ie play 4 + 5. I use that flat half tone valve significantly more frequently than the flat whole tone valve

What horn is it? A longer 4th slide might be something to consider; I believe that was a somewhat popular setup on small 4 valve Conns.