@vayu.rw here’s how I set mine up originally in the home and closer to the motor sag conditions:
the bungee is tight when the sled is in the home position and will loosen somewhat when it goes up, but not too much. notice the screw just to the left of the motor. If I take the chain off the motor, it hangs on that This was my first frame and it no longer exists. it took up too much space in my garage, so I made one on wheels and then I made the one now that folds down from the ceiling.
Here’s how I have it set up now:
both chain configurations are under. The first setup has the bungee that I shortened and wrapped from the chain around a pulley and over back and behind the motor. It would sag when the sled was in the upper corner close to the sagging chain. It skipped one tooth only one time of the many things I cut with it. The second one has as hanging 3 lb weight. It has wrapped a chain once when at the upper center position, and if the bike is in the wrong spot it will catch on it. but other than that, seems to be fine. The second frame is s 4’x6’ cutting area, so is smaller as well. You can really do whatever as long as the chains can moved freely and maintain tension of about 3-7 lbs. Too much and when the motor switches direction, the slop in the gear lash in the motor may cause inaccuracies.