If you have a reasonably accurate speedometer and tachometer, you can figure out your ratio.
Seems like a guy could paste a cheat sheet on his tank that shows rpm vs speed in the various ratios, until he trained his brain to just know. The bottom line is that if the engine isn't in the rpm range that you want, you shift. To my way of thinking, the only one position really need to be sure of is neutral. I get a false neutral position between 2 & 3 on my 2012 C5. Starting in 2nd can be embarrassing and hard on the engine.
I thought I needed a tach. on my C5 when I first bought it, but quickly realized that the sound of the engine is all the tach. I really needed.
I'm sure not enthusiastic about a gadget that counts shift lever pushes. That seems unreliable as can be. A gadget that is giving wrong information is worse than no information. To reliably know what ratio was selected on a 5 or 6 speed gearbox, I think you would need at least 3 micro switches INSIDE the gearbox, plus a simple logic circuit.
To my mind, these gadgets, even from the factory, are just "sizzle" to sell bikes to the uninitiated, and one more thing to break, one more thing to take to the dealer for repairs, and one more thing to buy expensive replacement parts for. I don't like power windows or locks in my car for the same reasons.
To know what engine rpm is preferred, it's worth taking a good hard look at your engines torque and HP curve. For boat engines there's a 3rd curve, which gives fuel consumption vs. rpm, but that's found experimentally and only applies to a fixed ratio drive train and assumes consistent resistance ( not variable winds or currents).