well, realistically, on 93 octane I run 12.8:1, and I have a couple close friends in Cali that run 12.2:1 on 91 octane, so I guess it depends on how conservative and safe you want to be. On on 91 octane to be on the safe side, I wouldn't run any higher than like 12:1 compression, maybe a tad higher, but keep in mind, the lower octane and higher compression, the more timing you need to back off.

In any case, good luck,
Aj