Max Lux happens on a totally clear day, at noon, on summer solstice. Every other day max lux scales down based on sun altitude and azimuth. My max lux readings are about 12000 in the winter, and upwards of 30000 in the summer. Depending on location, even max lux at solstice will be different depending on what latitude you live in.
I don't see where that matters though, you are just looking for a lux value that is bright enough where you want to change the brightness of the light. Those should be pretty constant lux values, though it will depend on how the sun comes in the window at different times of the year.
You might be better off with a lux sensor in the window facing out, as that would tell you how much light is actually coming into that room through the window.
You may want to play around with my SunCalc Illuminance Driver, it may have some features you could use.