I didn’t know this! This is a brilliant way of solving this issue.

A digital gauge would be best but I can imagine that the board might have a slightly different angle then the play field itself.

This old DA solution would be the best solution in my opinion. And you can even attach the two levels in a way that you could calibrate them with screws on both ends. This way you could even make your own preferred incline of 9,5 degrees (or whatever) the centered and ‘level’ position of the air bubble!