This question always brings up strong opinions on the correct way to play Bb on flute. We each have our preference. My suggestion is to get outside of your comfort zone. Why not branch out and try the opposite way? If you always play scales with 1 & 1 Bb, try playing them with Thumb Bb instead. Or if you’re a Thumb Bb person, try with 1 & 1 Bb. Go slowly at first. It’s hard to change a habit you’ve had for years. Try this with F, Bb, Eb, Ab, and Db scales. (Note: You should neveruse Thumb Bb on Gb scale nor on Chromatic). See how much your technique will improve if you have more options.
Thumb B-flat or 1 &1 B-flat?
I confess, as an undergrad flute student, I did not pay much attention to my teacher when she had me try this. It was difficult and I didn’t see the point. Then one summer, I tried this method and found it gave my playing so much more versatility than I’d ever had in the past. She was right (as always!) So now, I practice both Thumb Bb and 1 & 1 Bb scales. It’s amazing to see how much easier my students can play technical passages when they practice this way.
Further Challenge: Once you master both versions, try them all over again with the A# lever key.
Good luck! Happy Flute Playing!