You can calibrate the Shelly so that it knows the position of the curtains and this allows you to open them to a partial position, but it doesn’t actually know the current position, it just guesses based upon power consumption of the motor. See here - Redirecting...
I use Node-RED to keep track of the theoretical position (only open or closed in my case, no mid-position), but this isn’t flawless, it will be out of sync if I open the curtains manually rather than electrically.