I think references to the Fohn effect are confusing the issue even more - it is an ATPL subject.
FWIW, the Thom method of calculating cloudbases is incompatible with the exam as it excludes the SALR. The PPL confuser provides the exam compatible calculation...both of which are slightly different from the ATPL!!
The 90+% exam method is to read Thom, then go through the section in the PPL Confuser thrice.