There are a a lot of variables here - but typically pump gas tops out in the 11 or 12:1 range.
The basic guideline I have seen (based on static C/R) is 200psi cylinder pressure requires 93 octane. If you go up or down by 5 psi you need 1 more/less octane number . . .
Being in Nebraska you have E85 at the pump so you can blend your own higher octane. :-)
