How about temporarily hooking up the tank lights to one of the Z4K's track output connections (instead of the accessory output) and putting a meter on the connection at the lights and push the handle up slowly until you get the desired lighting effect - making sure the wires don't get too hot and then finding a power source that puts out the same voltage as shown on the meter when it gets to just the right brightness for you.
FWIW, I travel by a tank farm in East Boston and the lights on the tanks are not what I would call "bright" - prototypically, just bright enough so low flying aircraft can see them (not too far from Logan). However, for our train purposes, we probably want the lights a little brighter to show off the tank and the name on the side.
