questions on timing an engine

i'm thinking that since i'm running premium fuel having 42 degrees total timing is about right on 91 octane fuel. if its not i can set mechanical to 30 wich will lnd my initial to 5 or 6 degrees wich would give me a total of 35 or 36 degrees total timing and i could go back down to 87 octane but the book that came with this engine reccomends running 91 octane because of the compression ratio wich is 9 to 5 to 1 i believe?