What is your machine doing or not doing that points you toward insufficient belt tension as the issue? There are quite a few issues which initially appear to be belt tension related but are actually fixed elsewhere.
I went back and looked at the spreadsheet I used to calculate that table and, you’re right, the frequencies should be different for the 15mm belts, they should be quite a bit lower as the belt width lowers the resonant frequency for the same static tension.
For the Pro XXL, assuming 500mm measurement distance on the long axes I get 75Hz being good for ~150N on the belt which is more than enough.
The minimum belt tension is set by the maximum holding torque of the stepper motors, which I believe is the same on the Pro as on the SO3. This is, in any case, well above what’s really needed as you can’t sensibly reach stationary holding torque on a motor when the machine is cutting as the motors are moving and the faster they’re moving the greater the torque falloff.
The maximum tension is set by the 6.35mm shaft on the stepper motor, I was not able to find good data from the OEM for the Shapeoko stepper motor but the better (Taiwanese, Japanese or USA) manufacturers of motors with 1/4" shafts all give a maximum radial load that the shaft and bearings are rated to. Twice the belt static tension at 150N is already a bit borderline for even the good motors.
The Pro has wider belts which moves the load point further down the shaft, increasing the leverage. One of the key failure modes for excess radial load is brittle fracture after work hardening of the stepper shaft.
I’d be wary about adding tension to the belt on a Pro as I’d rather avoid adding my own set of pics to this thread
HTH