half a tank would be basically ~89, but if it marginally improved your situation, then either you have chitty fuel normally or some detonation, in my opinion.
Check your plugs and see how the gap is. .050 or .051 I think is OE.
Also, I know its been super hot here in AZ, but as the weather cools, you probably got a little more head room since the air temp will be better.
(I dont know the WA area, AZ we only get 87/89/91 unless somewhere special)
2014 Focus SE Manual -
2006 Focus ZX4 SE Auto -commuter 110k /traded