You should use a tension that allows you to wear the device for many hours daily. At first, generally the device should be worn at even less than your NBPEL; when you can wear the device without any problem for say 4+ hours daily, then you stretch a bit more, say few millimeters, and try to become accustomed at that tension. You should always keep in mind that you want to maximize time wearing the device, not force - if you want to apply some force, than you should go for manual stretches or hanging.
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