I am checking my links Pascale..one is explaining an easy way with a graph...I'll find it and get back...But your way with spot metering is the right way...and with dark areas..you have to shot for the dark as spotting an area that you want to be gray (zone V) compensate for the exposure an then get some more white in post prod (or darkroom)...though expanding the tone range...It's a question of seeing in black and white in terms of tones you want...then decide youself what will be grey zone V...wich the spot metring allow you...Mais je ne comprends toujours pas alors pourquoi utiliser la carte...comme tu dis...;-)