[Aavso-photometry] how cloudy is to cloudy??

[mlfleenor at charter.net]

Being new at this I am tempted to shoot even with thick cirrus clouds and "sucker-holes" should they open up later on. Right now its clouds and the forecast says partly cloudy with fog for the a.m. 

I have read that differential photometry is well suited for less than perfect skies but I am curious how cloudy is too cloudy??

Thanks,

Mike