Check my flows with lux sensor here:Lux sensor with continuous reporting
They switch the lights, but sunscreens should work the same.
1 make sure to pick a good spot for you llichtsensor. It should not meause the light inside the room that will be blinded. Otherwise they will open right away.
2 make sure to add the offset otherwise the screens will open/close with every cloud.
3 if the lux deviation is too sensitive you can add a timer. The screens would only open (or close) when the lux has been below/above the treshhold for a longer time
Good luck!
Dont hesitate to put your flows here, theres always help at the forum 
