Hi Paul
I used a Multigrade 500 set up for years when I ran a commercial lab. The system works pretty well once you have calibrated the probe - it will certainly get you into the ball park in terms of correct exposure, but you do need to rely on a degree of experience to know when to change metering points.
From memory you could meter from the black area on a final print (that is - a clear area on the neg that would print black), or from mid greay (the same grey as a grey card) - it just depended how you set the probe up. I seem to remember using the grey method for metering...
I would recommend getting hold of the Multigrade 500 Instruction book, as it's been so long I can't remember how to set one up. From memory it came with standard negs you used when setting up the head.
These heads are excellent units, and make printing (expecially split printing using multiple grades) very easy. And if you don't already have one get hold of the foot switch - they make printing so much easier..
Cheers
Andrew