I use my spotmeter with a reverse mounted 50mm camera lens for 35mm and a small light table with a sheet of black construction paper w a hole cut out.

EXACTLY..... But why the reversed lens for 35mm? My experience is to shoot a Z-I and any other Zone I feel I may need on each roll of film and measure that/those entire negs. Does the reversed lens allow you to pick out a small area of the 35 neg to measure?