I'm sure your close to figuring out your solution, I am just asking this for my own understanding. You are creating logic that reflects where the part is at on the conveyor. If you know exactly how many seconds it takes for the part to travel from one end to the next, could you not create a simple timer and use the .acc value to move the image on the screen horizontally relative to the time through the HMI software?
I have a process where a part is spinning on a fixed enclosure, and on my HMI I am using the exact degrees from the PLC to locate the part on the HMI screen, so the operator knows exactly what direction the part is facing at all times through the screen.
This is my way of doing it, maybe I misunderstood what you are trying to accomplish. Or maybe you are looking for more accuracy. I just ask for my own understanding.
Thanks,