as I understand Mika isn`t here for a long time.
I try to understand your problem:
You want to steer the position of the lamella angels. so Shutter is DOWN (Dark) and just turn the slats a little bit for getting more light.
The parameter open/close I think is only for complete open or close.
I have 6 of the ShutterControls, but work only with the “level”.
I cannot test my suggestions, but I would say:
Parameter 10: 2 Venetian Blind Mode, with positioning
Parameter 12: Default 150 -> In Venetian Blind mode (parameter 10 set to 2) the parameter determines time of full turn of the slats