Producing Isostatic lines of principal bending stress using Millipede

Hello,

I was trying to use the millipede plugin as a basis to generate isostatic lines of principal bending stress. I have attempted to do so using the stress lines component of millipede. However, theoretically speaking isostatic lines should not change when the scale of the load applied is changing as the lines only represent the direction of principal bending stress. I am wondering if you have ever faced that issue, and if you know how I could achieve true isostatic lines.