Sonic measurements will vary, influenced by mic sensitivity and positioning, so unless you’re building a test rig that incorporates a placement jig for the phone mic, your measurements aren’t any more accurate than simply ‘pressing down’ on the belt.
Step 1.) Slide the moveable component, gantry or carriage plate assembly to one end of its travel path (the following test works on either belt).
Step 2.) Place your thumb and index finger on either side of both halves of the belt, approximately 2" away from the pulley wheel at the opposite end of the component travel path.
Step 3.) Apply moderate pressure to the belt, about as much as would be required to lift a half-full can of soda using the same fingers, you should be able to deflect the belt about 1/8" on either half, but not touch the belt together without increasing the amount of pressure.
Step 4.) Release the belt and move farther away, to a position 3" away from the pulley wheel, and perform the test again using the same amount of pressure. At this position, you should be able to just touch both halves of the belt together, without having to increase the amount of pressure from your fingers, if so, your belt tension is correct.
If the test does not follow this progression, adjust the tension either up or down to achieve the correct progression.