Best bet might be 2 concentric annuli
1. Current search area i.e. based on the 7th arc. This is the "AP, fuel starvation, spiral scenario."
2. Based on maximal glide (from cruise) distance beyond the 7th arc. This is the "piloted flight, get as far as possible, controlled glide and ditch scenario." Most likely FL350-400 if trying to get as far and fast as possible. If trying to disappear, it would seem illogical to reverse course during glide.
This strategy may be more productive than searching all points between, for which seem to be arbitrary in that there is no scenario pointing to these areas. And I get a sense we aren't going to increase the search area by orders of magnitude. Any further search will need to be relatively focussed.
This of course assumes the BTO rings are accurate. Some have speculated less accurate at low temperatures. However a piloted flight for approx 7 hours wearing normal clothes would require environmental control.