Heart attacks are fewer in areas of hard water, unless you count those caused by stress due to problems with developer.
Chlorine, fluorine and oxygen in water can cause loss of activity in ascorbate developers. Excess sodium ascorbate or sodium erythorbate (aka sodium isoascorbate) can cure it.
I have a Leica M3 with clip-on meter that would probably work. I'm not familiar with your meter. The object is to measure the output of the lamp, which can be done consistently by making the bellows as short as possible, removing the lens or lens board, and placing the meter right at the opening.