Rob
From memory, I did a test strip to detemine exposure then useing the same appeture placed the defusser against the lens and adjusted the dial till the neon light just whent out. This gave you a reading for the paper speed, if I recall these are the letters.
On the next neg using the same paper set the paper speed and again holding the defuser against the lens turn the dial untill the neon light just flickers off, then read the exposure.
I found it best to turn the safe light out when takeing readings.
Have to say I went back to test strips as they gave me more information, but the meter was useful when I had a lot of negs to print. These days I do little 35mm and I never used the meter on med format negs.
Regards Paul.