This was always a problem on the 1-11, we had simple drift down tables, I also seem to recall a requirement to make a note of your single-eng level off height on the plog.
As far as a decision point goes, well thats down to you like most things in life ! and so much depended on the day. There was a chart floating about,(think I may still have one) with beacons and various radials and distances, which used to show the best route to fly during the drift down, I don't know who produced it but I think simular charts have been constructed in the past for route aproval from the authority.
Best regds