I'm wondering if you're using your cold light (with the heater) and you set a base exposure of (say) eight seconds, followed by a couple of three second 'burns', how will the head have time to warm up and stabilise? (I know these are short exposure times, but they give reliable results using other (condenser, diffuser) heads I have.
And then, ten minutes later you want to test another area of the image (same base exposure). The head would have cooled down by then....I just thought - maybe the heater is left on in the head during standby?? ..could be wrong.
And would this be a typical exposure regime in your darkroom, giving consistent results?
I can see that the compensating timer would work well here, but I'm not ready to spend more money on such a timer yet.