I will consider it but another blueprint may do what your after already.
Have you seen the 
Sensor Light blueprint. It will turn you light ON when presence is detected and if you use the ambient condition the light will only come ON then presence is detected and below you lux setting. If the lux rises then the light will turn OFF for you.
It is not a manual turn light ON but thinking of the logic. If I was to add a lux sensor option to this blueprint, if there is proper sunlight and the light was turned OFF then you would have to manually turn the light back ON if later in the day there is not proper sunlight. The sensor light blueprint will do this automatically for you and has a low and high setting with time delays to prevent the light going ON / OFF on those cloudy days.
Blacky 