What is the normal voltage level for a battery in a 6 cylinder falcon?

While car is turned OFF: ??

While car is turned ON: ??

12ish volts, doesn't matter if its not exactly 12, mines about now 11.8.
And I've never measured voltage when my cars are on.. but it would depend on how long the alternator has been charging it for and how much charge it was receiving.

Keep in mind that a 12 volt battery doesn't get charged at 12 volts.

Off: around 12v. If it's less than 11 the battery probably has a dead cell, and is a throwaway item.

On: 13.5-14.5v. If it raises and drops (a lot) with engine revs the altenator regular is dead, and you can be cooking the battery at high revs.

There's no exact figures.