can download a great little application called NDCalc. This will do the calculations for you and provide you with a count-down timer. Taking into account reciprocity failure (for long shutter speeds), I will often double the time. 1/1000 1secs 1/60 16secs 1/4 4mins 1/500 2secs 1/30 32secs 1/2 8mins 1/250 4secs