Ah yes....when you're on final and push for managed speed and the bug goes way up there instead of somewhere near approach speed while those engines come back to life, you know that you should have activated...
Our manual tells up to activate somewhere between 18,000 and 10,000 and when managed speed is no longer required, since the LAX ATC will have a field day with you anyway(slow down , descend, go fast....), forget about that wonderful precalculated descend path!! Activating the approach phase really only changes the reference for managed speed. Has nothing to do with your ILS or anything, just where the speed bug goes when you use managed speed. And if you don't do anything and fly your descent/approach in NAV mode ( a managed lateral mode, to include LOC, but not on all A320's out there) , 'Fifi' will automatically activate the approach phase when passing the DECEL waypoint (and it will never be above 7200' AGL, by the way). And since every approach environment is different, they just give us very few guidelines and leave it up to us to screw it up :-)