I don't know if the following link may help answer your question Hans, it's beyond the capabilities of my pathetic brain (any more than 2 syllable words are beyond my comprehension) but it is an interesting read on cloudy nights discussing much the same and a couple of other interesting links:
https://www.cloudynights.com/topic/8...by-wavelength/
https://www.cloudynights.com/topic/8...higher-strehl/
http://www.rfroyce.com/standards.htm
I only have achromat refractors, I'm more concerned about street light ratios than Strehl ratios.