Originally Posted by prj71

You are complicating this more than it needs to be. It's a physical characteristic that you can feel with your legs and lungs. Knowing how fast you are going or how many watts you are putting down is much different than the physical feeling in your legs and lungs.

If it gets harder...shift to easier. If it's too easy...shift to harder. When you run out of gears either work harder or get off and walk. If you run out of gears going down a hill..start coasting. Doesn't matter what gear you are in...just shift whenever necessary. Gear indicators quickly become irrelevant to riders as they gain familiarity with their bike.

I've had them in the past on bikes in the 80s and 90s and that was with 3x drivetrain...never used them.