Control Mackie FLIP button with general midi

I guess it's the Mackie FLIP button you mean.

Mackie buttons aren't 127=on, 0=off; they work as a computer keyboard, i.e. when you press the button it sends 127, and when you release the button it sends 0.

So there is no way to set it ON; you can only send a 127+0 combination to toggle its state. If you want to make sure the state is the expected, you need to keep track of the current state yourself and only toggle if it's wrong.

Yes, note 50 is correct. I forgot to set the correct middle C when checking the note number. It's D2, D3 or D4, depending on how you set middle C. So it's a bit dangerous to use note names.  

The flip button is in the daw-specific area, so the note number is probably daw-dependent, even if it seems pretty standardized.