Hi Mutt! The charts we use in the 747-400 have certain very strange characteristics. For example, in certain circumstances, as you increase the depth of contaminant, allowable takeoff weight increases. This leads to the theoritical case where the flight would have to be delayed til more snow covered the runway. I bring this up during recurrent each year; everybody scratches heads, then we proceed to the next subject.
Do I trust these charts? Right up front the FAA says they are not 'certified' for use. There is only one right answer...wait for the plows!