Ford's OBDII system uses the O2 sensors to measure and then compensate for the higher alcohol content by adjusting the target AFR. This should eliminate the possibility of operating in a lean condition.
If the R+M/2 value is truly 105 (there is considerable debate on this true octane value), you should be able to advance the timing up to 6 degrees (this is the theoretical max based on 1/2 degree for each increase in octane; [105 - 93] / 2). <---Someone please confirm my memory!
Obviously you should make small changes until you detect knocking. If you advance too far (too quickly) the knock sensors will dramaticlly reducing the timing advance.
The relationship between PM and AFR is shown in the table below.
Stoich Air Fuel Ratio = 14.64 - 5.64 * PM
PM (percent alcohol)...Stoichiometric AFR
0.00 (100 % gasoline)...14.64
0.05... 14.36
0.10 (standard gasoline)... 14.08
0.15... 13.79
0.20... 13.51
0.25... 13.23
0.30... 12.95
0.35... 12.67
0.40... 12.38
0.45... 12.10
0.50... 11.82
0.55... 11.54
0.60... 11.26
0.65... 10.97
0.70... 10.69
0.75... 10.41
0.80... 10.13
0.85 (standard E85)... 9.85
0.90... 9.56
0.95... 9.28
1.00... (100% ethanol) 9.00
If you intend on using E85 all the time, I would recommend you discuss this with Bill (if you haven't already). He might be able to write your performance tune based on E85 (or even write you an E85 performance and towing tunes since the CS and CTS are capable of storing up to 20 tunes).