The longer the better. The goal of the spring is take out any slop in the coupling nut. The longer the nut the less slop they typically have. I have been messing with it I haven’t found any real need for this all the forces tend to be in one direction consistently. When I designed this I thought using it as a mill would push back on the whole thing turns out it pulls on it in the same direction as gravity, meaning slop isn’t really an issue.
I have learned a lot since I started this. I was pretty sure wood would be out of the question, turns out even aluminum works well.