Elliot,
I can't say what is optimum, but I have been experimenting with my studio, and what seems to work for me is heavy drapes to control outside light, and halogen artificial light at a moderate brightness. When I calibrate my display with my colormunki, ambient readings are around 50/55 lux.
Under these conditions I get pretty good results in terms of print to screen matching. I am planning to get a few Solux lamps on a track to get a bit more light on the wall where I do my print viewing, which I think will get my visual match to my monitor even closer.
Overall I like to have enough light to be able to properly evaluate the prints, and I try to keep the brightness of my monitor at the value suggested by the colormunki during calibration.
Stefano