For test strips, I normally use 8 seconds, 11, 16, 23, and 32, mimicking the f stops on the lens. That means if I have a 23 second exposure and want to make it 32 seconds for more time for dodging and burning, I just close down the lens one stop. I also have a chart on the wall with full stops, half stops, third stops and quarter stops, in case I want to dial it in. If I want to make a larger print, I can map the exposure in stops.