I do not put all that much chemistry in. I usually put the amount
recommended by Jobo.

I would feel more comfortable if I could fill the water jacket with water up
to just below the filler hole, but that is way too much and floats the tank,
even with reels and water in the same volume as the chemistry would be in
the tank. Actually, I would prefer the tank to be completely submerged.

I once talked to someone at Jobo (Ken Owen???) and he said it was sufficient
to have the water as deep in the water jacket as to come up to the level of
the chemistry in the tank, so that the water was in "contact" with the
chemistry. Well possibly if the tank were stationary, but in fact it is
rotating, so I imagine it is all over the tank inside just as the water
would be all over the tank outside if its surface tension were lower. Hmmm.
Should I put PhotoFlo in the water jacket? (Probably not as it would
probably gum up the heater.) Because the water on the drum outside the water
bath probably evaporates and cools it.

Now as a practical matter, the water bath works very well at holding the
temperature inside. I probably measured this (using water) way back when,
but I cannot find where I wrote down what happened. Probably because it was
not surprising. I normally run the water bath about 1/2 degree F too hot
(once the solutions are up to temperature) for some reason (that I also
forget). I assume this is to compensate for falloff during processing.

Of course, for black and white, this matters much less as processing
temperature is about the same as summer room temperature.