[Leica] black and white

["Jeff S" <segawa@netone.com>]

Maybe some of you darkroom wizards can help me with this one:

Before heading to China earlier this summer, I purchased 200' of Tmax 400,
ran film speed tests and determined that the optimum speed was 100.
Unfortunately, upon my return, I found I had made a number of mistakes
during my testing, mostly due to the fact that I had not read Kodak's XTOL
data sheets more carefully.

- -I had processed the test negatives in a 1:3 dilution of XTOL, but in my
Jobo tank, this means that there was only about 125 ml of concentrated
developer for 2x rolls of film--well below Kodak's recommendations.

- -With these mistakes corrected, it now seems as if ISO 400 is about the
optimum film speed, though my results seem to vary some from batch to
batch-not sure if this is due to my exposure technique (below) or
processing. (should I get a Jobo CPE-2, even though it looks like a $500
dishpan?)

So, my undeveloped film would seem to be overexposed by two stops, and I'm
wondering what my best course of action is.

I know from Fred Picker's book, how I determine optimum film speed settings
and processing times. But how does one determine optimum processing
temperatures?

And this is probably a dumb question regarding the M6 meter, but...I did
some Zone I - Zone V test exposures using the black side of my focusing
cloth as the target, and later did the same, using the white side (both
times, under open shade conditions). I metered very carefully, but the
results are not the same! I'd have thought that, regardless of the surface
being metered, if one lights up the two LEDs, that's exactly Zone V, period.
Am I encountering some sort of nonlinearity in the meter's response, and if
so, should I re-do the film speed tests using a standardized target, such as
an 18% grey card?