I have had a look and if you are getting prints like these then I can't see anything wrong with your exposure method. If you were to move to a 5 degree spot attachment would this be small enough to take a reading on the right area? Caucasian skin is said to be Z6 so a spot reading needs an adjustment - possibly.
Not strictly relevant but I think your B&W portraits are better than you rcolour ones. I'll re-phrase that. They had more impact for me