Thanks a lot for your answers ...
Unfortunatly i don't have any still digital camera (will get one soon).
So, basically, the best is to take a incident reading when teh sun is behind the clouds and expose for that,? ... just afraid that when the sun will comes out of the cloud it will be completly burn out (totaly white frame!)
PS :
Sorry about saying to take a spot reading directly towards the sun, this wasn't what i wanted to mean (I am french and my english is quite poor, especially when i write too quick!!) ... but taking a reading of the clouds when the sun is behind.