Sparkles or handheldfirework

Tonight I've tried out the Bulb (B) function.
For darker background, ISO 100 seems to work best, but I am still trying to figure out how to calculate the Bulb value.

f/5.6  1 sec  ISO 100  exp 0  70mm Bulb=3.0

f/5.6  1 sec  ISO 100  exp 0  70mm Bulb=3.0

 f/5.6  1 sec  ISO 100  exp 0  70mm Bulb=5.0

f/5.6  1 sec  ISO 100  exp 0  70mm Bulb=9.0