No Rule
There is no set rule about octane and compression... It is generally accepted that any engine under 11:1 can run on 94 octane and lower... Also engine requirements change as it gets older or how much carbon has built up (Carbon will both increase your compression a bit and keep the heat so it will be easier to pre-ignite.). The only for sure way to give your engine what it needs is to fill it up with a tank of the gas you have already been using, keep a log of what mileage you filled up at and how much you put in, and go for a drive... Does it feel like you have the same/less power than before (Better if you have a GTech pro

) and mark the mileage down... Then repeat with different levels of octane until you find the right one. I would also tell you to listen for pinging (a sign of pre-ignition), but I think your car may be equipped with knock sensors and you won't hear any because it retards the timing to prevent pre-ignition.
Sorry it's such a long explanation, but I hope it helps you.... It won't hurt the car for the short period it takes...