I can dig up my notes if you are at all handy with DIY electronics fiddling.
You place a potentiomenter across two of the front plug in terminals, and then you can dial it back to anything as little as about 1/12 power repeatably.
I just soldered stout wires onto the wiper and one track lead of the pot, and bent them to suit, and plugged them in.
It stayed on the front of the unit from the friction fit of the two wires in their sockets.
I added and old knob with pointer to the pot shaft, and fashioned a dial face.
I marked calibrations on the dial face after firing the thing into a flash meter a number of times to figure out what the varied settings on the pot were giving me, light output wise relative to full manual output.
All up about 1 hour of effort, and maybe $3 at the electronics wholesaler.