Which light meter/sensor are you using to measure light on your window sills ? I have downloaded 3 free Android apps but they each gave different numbers on the same window.
How far off were those numbers? Even repeat measurements with professional photographic equipment has some error. I use a Sekonic 558L light meter, which I already have for my photography. At around $600 not something you buy on a whim. Even there, you have to know how to use it, or you get wrong readings.
Phone apps won't be accurate unless the programmer had specs for each kind of light sensor in every phone. An inexpensive photography light meter will probably work better than a phone app.
__________________ May the bridges I've burned light my way.
I use an Android app that allows you to calibrate the sensor manually.
I've calibrated it using my SLR.
It is sufficiently precise and consistent and allow me to make an assessment of the lighting conditions.
Together with a careful observation of the leaves color, it has proven useful.
Ciao Nicola