For the BG Scroll action setting a formula to determine the CENTER attribute is flawed in a couple ways.
The Formula Editor states, "Must return one of: CENTER, FIRST, LAST, SCREEN1, SCREEN2, SCREEN3, SCREEN4, SCREEN5, SCREEN6, SCREEN7, SCREEN8, SCREEN9, CENTER_M1, CENTER_M2, CENTER_M3, CENTER_P1, CENTER_P2, CENTER_P3"
This is incorrect. I haven't tested them all but using a formula like SCREEN$si(screen)$ returns SCREEN1, SCREEN2, etc but doesn't work. I have to use Screen $si(screen)$ to return Screen 1, Screen 2, etc then it accepts it as a valid entry.
The second half of the problem:
Using a formula like that doesn't work. It sets the center to screen 1 always.