In addition to @jepho’s (future) input, here’s a thread describing a process to set belt tension in a “scientific” way
The “belt tone” method boils down to downloading the Gates app on your phone, move the gantry to one side, insert a 1/4" dowel under the middle of the belt, pluck it, adjust tension until reading ~100 to 120Hz
What’s important beyond absolute belt tension value is to ensure equal tension on the left and right Y belts.