Set - Check - Adjust.
A safe bet is to find the tension that begins to drag down the freespin of the system (on the bench) and call that 'appropriate'. When you apply a load the belt stretches just a little bit, so that extra drag goes away durring operation.
Also, looser belts not only skip, and/or roll over, but they subject themselves to vibration and getting torn up on nearby items.
Jim Herrmann, ex-Professional (turned Life Coach)