I bet you'd be just fine with 100. A couple weeks ago I had to mix 110 and 91 about 50/50 and it got me by. Not say I'd do it every weekend, but it worked.
Do you have a dyna?
Keep in mind that the 12.5 more than likely is'nt a "true" 12.5. Also if the Kenz grind cam is on the bigger side, that will help bleed off compression.