Well,the octane number mostly depends on your compression ratio,
here's a little chart I found floating around the internet:
CR Min Max
8:1 87 92
9:1 89 96
10:1 92 100
11:1 96 102
12:1 100 108
Typical requirements,some may vary.
Also,note that depending on your mods and parts,you can get "knocking" if you do get that,run slightly higher octane.
Compression is what gives the added power,not the higher octane fuel.Higher the octane,less "explosive" it is,kinda and so it resist the comp.
Hope this help.