I don't quite understand the level of water in the tempering bath ...

"Up to the `shoulders' of the bottles" seems appropriate ... I have *no* trouble holding that ... If you can't fill to that height; there has to be some sort of drainage somewhere. In operation, the rotating developing tank should have its lower 10% - 20% sloshing through the tempering bath - that should be sufficient to maintain the required temperature in the developing tank. If you can only get the fill level halfway up the bottles, you must not have any tank contact... ?

JOBO sets the bath temperature a degree or so above the LED readout to compensate for losses in operation. Can't prove it by me - I have *no* idea of a way to measure the temperature in the tank during rotation.