Velleman K8200 & 3DRag - Belt spanner for double Z-axis (Gesiima solution)

This belt spanner is for the genereal set-up as you can see in this Thing:

I had changed to 20T XL Aluminium timing pulleys, so in consequence I had to adapt the spanning mechanism or take a shorter XL belt. I am using currently a 506XL037 and would have needed to go down to something like 490XL037.

For the pulley I am using a bearing and have provided it with shoulders to prevent the belt to slip off.

Why did I do the change - even with best calibration a printed 24T XL Timing pu