You should just be able to tap the back-stitch button when you reach the length you want, then it should finish it and remember it.
However... sometimes the 3A foot needs to be re-calibrated by your sewing machine tech. Sometimes the those 3A feet "fail" to calibrate, and need to be replaced. If it needs to see the sewing machine tech, take both the machine and the 3A foot. It could be the foot's fault, or there could be a disconnected part on the circuit board.
I hope it's an easy fix.