How many valves does it take...

[Heliconer]

This is really a question of valves and how they're tuned.

1-4 are pretty standard.

1+2=3
1+3=4

But the question comes down to 5 & 6. My PT-6P has a 5th Tuned to Flat 1.
I've played a miraphone CC that had a 5th tuned to 2+3.
But what about the 6th valve on some F's? Maybe a flat 2?

What variations of this do you know/use?

[dmmorris]

Most common in my experience is:

5 = flat 1
6 = flat 2