I have read that without the lead additive the engine may knock
Keep in mind that the advertised compression was not the actual compression. Pontiac advertised 10.75 and 10.5, but this was with minimum volumes of the head's combustion chamber. The heads were generally more than the minimum and you had to mill them to get there.
You may or may not get the "pinging" or "detonation." The lead in the old gas was a lubricant for the valves as well. You can add an additive as bigD suggested or mix some racing gas in with the pump gas. You also may be able to adjust timing to get rid of the knock. Retarding the timing and/or changing the advance curve may work and you can still use the higher octane pump gas - but you can only retard timing so much as it will begin to run hotter, and you will be giving up engine power.
On ethanol, your engine may also run leaner and you may want to change the jets in the carb by going up a few sizes to richen it up a bit. Other things you can do is make sure your cooling system is in top shape, fan shroud installed, good fan(either factory clutch fan set-up or aftermarket flex fan), a 160 or 180 thermostat, and you might want to try one step colder spark plugs. Minimize the heat in the engine and it will help with the gas situation.
The problem with ethanol is that it goes bad quickly if not used up, absorbs water, and deteriorates the "old type" rubber hoses and even the needle tip in the carb if so equipped. Non-ethanol gas will last a lot longer if your car sits. With a gas stabilizer, it can be good for over a year or more from what I read.
So, I don' think anyone can give you an exact formula to use when selecting what gas to run - you may have to experiment. However, what you don't want is to allow the engine to "knock" "ping" or "rattle" as this will destroy an engine if allowed to go on for long periods of time. This usually happens under heavy throttle or under load and can be more common on really hot days as heat can contribute to the "knocking" or "pinging." If you do experience it, then back out of the gas so it stops. Then make your adjustments in either gas mixtures/additives and/or timing changes.
So it will be something you will have to play with. If you ever have to rebuild the engine, then you want to get pistons that will give you around a 9-to-1 compression ratio with iron heads and you should not have to worry about gas octane choices.