yes it is safe... at least as far as I have ever read. however i do believe since it lacks detergents that it can cause more soot buildup in the motor. but for once and a while basis it shouldnt be harmful.
HERE IS AN OCTANE MIX CALCULATOR
if you use xylene or toluene (i forget the octane rating of those) just use the calculator. personally I would rather use race fuel found at any race track. much simpler and I think cheaper.