I made a video about belt tension, it’s in this thread: X-Carve Maintenance/Troubleshooting Videos - Add Your Own! - #2 by RobertA_Rieke
My belts are set to about 3.15 pounds to lift up 1 inch in the middle. I got this cheap fish scale from Amazon: https://smile.amazon.com/gp/product/B00B301MPI/ref=oh_aui_search_detailpage?ie=UTF8&psc=1