Question About AT2100 Stepper Driver

Hello everyone, I was working on a project and wanted to ask a question about driver. I have explained it throughly down bleow.
In summary, I saw that the value reached to a maximum of 0.7 while adjusting the vref values in accordance with my calculations on the AT2100 driver that I used to control the 17HS1538-P4120 and 17HD4002H-22B stepper motors (but the vref = 1.3 and 1.5 I calculated). and in addition, it increases and decreases unevenly. What could be be the reason for this? I have given my purpose and the details for the project I have found down below.

I encountered a problem while designing the software of the cnc device. First of all, I connected Cnc shield v3 to Arduino and I use AT2100 drivers to control 17HS1538-P4120 and 17HD4002H-22B stepper motors. I had to do some calculations to set the Vref on the driver. Unfortunately I could not find any Vref formula about AT2100 on the internet. Due to the similarity between them, I made the vref calculation with the values ​​of my own driver using the Vref formula of the TMC2100 driver. When I added the stepper motors for your work, my Vrefs came out to be 1.7 and 1.5 respectively. When I want to run with ninety percent efficiency, my new values ​​are 1.5 and 1.3.

In order to adjust the Vref, I connected the multimeter to the screwdriver with the help of alligator and of course grounded the other end. While taking measurements simultaneously, I tried to bring the driver to the values ​​I calculated by turning the screw. But my drivers were up to 0.6 at the most. In addition, it was either constantly increasing or decreasing depending on the rotation direction of the screw in the videos I watched. But in my driver, this value increased and decreased irregularly even though I rotated it in the same direction. Can you help me on this topic?

The only datasheet I can find relevant to AT2100 on the internet: AT2100 Datasheet

TMC2100 Datasheet:
TMC Datasheet

The formula I use when calculating vref is as follows. A so-called formula characterized only by Imax. According to the datasheet above, the maximum current of the AT2100 is 2.5 A, but since the maximum currents of the motors we use are lower, I used the motor currents to base while calculating.