With the TA I had I ran 93 and started at 13. At 14 it would ping on hot days and be fine on cold ones. I have however seen people running tham at 16 or 17 with no detonation on 93. So, each car will react differently. My suggestion is to keep the allen wrench in the car at all times. If you get bad gas or something you can back it down immediately.
Your advanced spark timing requires a better burning fuel, or else you will ping all day long and probably end up with less power than you started with. Make the jump to 93. It will cost you all of 5 bucks extra to top off your tank