My advice would be as follows.

1. Wear a full body harness. It may not be totally safe but it is far far safer that just and upper body harness or even worse a belt.

2. Attach the tether strap as high above you on the tree as you can. This is important for two reasons. One if you attach it on the same level as your body and you fall out of your treestand, falling just the lenght of that tether strap and then being snapped to a jerking halt in mid air can break bones, permanently injury your back and damage internal organs. Secondly, by attaching it higher up if you fall you will be stopped at a level high enough to reach out and grab your tree stand to pull yourself back up.