Good to know! I just googled, and it looks like according to Cobb, it should actually be -1.0 optimally. Unfortunately there is no E85 within an hour of me :/Watch your "octane adjust ratio learned" (OAR). It starts at 0.0 and will move up to 1.0 if you use good fuel that is the correct octane. I'm no expert, but you might be able to get there on 92. Some people add a gallon of e85 for a little octane boost, so that may help.
This ^No, you cannot run a lower octane than what your tune calls for.