How about using a small digital spring scale. Hook it under the belt and pull until it’s a specific height above rail. The weight displayed will correspond directly with belt tension. You can find them for $10. Just a thought.
Personally I just push the gantry to the opposite end and make sure I can fit a finger between the belt and rail. You can easily feel differences in the Y. “Guitar string tight” imo is when it has a little twang to it. Its a feel/ear thing, same way we tension bandsaw blades.