True Fitness Treadmill ES9.0 + ES7.0 Owner's Manual page 65

Turn both rear roller adjustment bolts counter-clockwise until
treadbelt just begins slipping when walking on it, then turn both
rear roller adjustment bolts clockwise in equal quarter turn
increments until treadbelt stops slipping.
Note: Be sure to run on treadbelt to ensure that the treadbelt does
not slip while under load.
Treadbelt Tension
Chapter Nine: Care and Maintenance
ES Treadmills Owner's Guide
65