P.S. Better to lose the old mindset of mentally wanting to have gears numbered or named in your mind. Think big and small or instead of numbers.

Just think of the front small gear as not being needed unless you are going uphill in some way. Think of the middle front gear as good for most level riding.
Then think of the rear gears as being fine tuning variations for those front gears.
Then, it logically follows that you wouldn't want to be in your most "uphill" gear in back(biggest ring) paired with your most "downhill" gear in front(biggest ring). Or vice versa. Maybe I'm off the mark, but this way of seeing it helped me.