Originally Posted by NedL
Making a percentage test strip is easy. I like a 20% difference for my initial strip. I start with 10 seconds then cover a bit of the strip and count 20%, i.e., 2 seconds. The next strip should be 20% of 12 seconds, but that's close enough to 2 seconds, so I just give 2 seconds to the next bit of the strip. Now I have 14 seconds; 20% of that is really close to 3 seconds, so that's what I give the next bit. Now I have 17 seconds; 20% of that is rounded to 3 seconds as well so that's what the next bit gets. Now I have 20 seconds; 20% = 4 seconds here, so the next bit gets 4 seconds for a total of 24 seconds. The next strip will get 5 seconds to make 29 seconds, then the next gets 6 seconds to make 35 seconds total and finally, 7 seconds to make 42 seconds total.
Seems complicated, but once you figure it out you just count (with the metronome keeping seconds for you): 10 - 2 - 2 - 3 - 3 - 4 - 5 - 6 - 7 - off; easy and close enough to 20% so that the longer exposures have adequate separation - and you have a range of 10 - 42 seconds to choose from.
I think of all my dodging an burning in percentages also, and notate them as such in my exposure records. This is very helpful when scaling a print up or down. One simply finds the correct base exposure and then calculates the dodging and burning using the percentage of the base exposure. Of course, this is only a starting point, since different size prints almost always need some adjustments in this regard, but it is a very close starting point.