Quote Originally Posted by mhv View Post
One thing to bear in mind with bulbs, is that even when used with leaf shutters, they don't have a constant guide number between shutter speeds.
The best resource I have found to help with this issue is the 1970's-era Kodak Master PhotoGuide (the blue cover book). There is a calculator that helps sort out the myriad of variables - bulb type, reflector type, shutter speed, distance, and aperture. I've found it to be "dead on" 99% of the time.