OK, I have Shimano SH-R443 Rapid Fire shifters on my road bike, they work the same as MTB shifters. They have a single line indicator moving across an arc with two bars in the middle of the arc.
I got some lettraset or similar and put the numbers 1 5 9 on the body of the shifter when I was learning which way it all went. It you have a VCR cssette, the labels have sets of useful numbers in the box for labelling the casette, that way you can label it which ever way you want to go. 1 5 9 or 9 5 1. 🙂
For me "left" on the indicator was always "smaller", however a Rapid Rise Rear Derailleur would be the opposite. It would give you what you want.
I don't advise this, when double down shifting, using both index fingers is a real advantage.
I came to cycling from a motor cycle background, one of my bikes had the spark advance on the left twist grip and the throttle on the right, I wound them together, not opposite. 😄