I've had a couple of these fixed-everything cameras, and it seems like f/8 or f/11 and 1/50 is a common range for the settings. In full-sun conditions, I usually shoot a 25-speed ortho film in them and develop in Diafine for compensation; ASA 100 for brightish-overcast, and I suppose 400 would work for dimmer conditions, but I don't really trust my ability to eyeball the light levels at that point. In any case a compensating developer is pretty useful.

Marcy Merrill says good things about it: http://junkstorecameras.com/ImperialDebonair.htm.