I know that decision height must be at least 1.25 times bigger than your minimum A/P disengage height. The airport DH is given as 207 feet (which is more restrictive than the A/P off height x 1.25 for this exercise).
Here they ask for that minimum A/P disengage height and I do not know of any regulations asking for a safety factor. I would be very surprised if any ATPL GS books even mention anything about min. A/P disengage height.