Phil,

Three pathways I can think of:

1. Just accept the limitation. If you ever get to watch Allen Shelton, one of the all-time greats, you'll see he extends his fingers a lot more than necessary/Earl/efficient. But it sounds good, so he just kind of chuckles about this. But you say it hurts your sound.
    
2. Play as fast as you're able to play *without* the problem happening. Note what speed the problem starts happening at. While playing at that speed, stare at the middle finger and tell it not to DARE stick out. See if you can mind-over-matter it like that. If you can increase the speed and keep the finger behaving, see if you can keep it up without having to concentrate so much. If you can do that, that's "good new habit development". Stay at this threshold of when the problem starts to arise, and see if the concentration helps you keep it controlled. If so, try less concentration and more speed, a little at a time. You might be able to reprogram the finger that way.
    
3. Another more physical way of reprogramming: The clarinet player in Flexigrass, Bill Pontarelli, developed an apparatus whereby if he extends his fingers, they encounter actual restraints. To avoid running into that, his fingers find a way to move within the closer limits. In time, he had developed new habits, extending his fingers less.

Hopefully one of these choices will work for you!

Pete Wernick