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I am a bit confused: option one is playing Bb + Cc, option 2 is playing Bb + Eb - why would the Eb version be easier for you to finger than the Cc version ? The only thing you need to practice with the Cc version is a transposition of one tone. That is easily done.

Whenever you practice any tune, you just play it thereafter one step up. Or down. Every day, just for 5 min. After a month you'll do it without thinking. Don't think over it, just start with easy tunes and do it. Your brain figures out the math by itself. Easy with a bit of persistence.

I think people's minds do not all work the same (brilliant theory here, take note!) For me, the difference between one (whole) step up and one fourth up is, the first is a little easier to mess up. I don't do arithmetic while I'm playing, anything like that will jam up the works. I can read and hear intervals, at least a little, and two distinctly different possibilities makes the easiest choice.

But I'm not here to advise anyone to mess around with more keys of tuba than they want. Anyone who would do that because of advice on Tubenet doesn't deserve to have paying gigs.