I use a metronome too, or at least the metronome function on my GraLab 450 timer. The timer itself is set to 99 seconds, the highest it will go. I begin and end exposures by revealing/blocking the light with a card; hold the card over the lens, step on the footswitch, remove card and begin counting. I count up to my target exposure like a musician counts beats: 1 - 2 - 3 - 4 - ... - 15 - 16 - OFF, so I never lose track of where I am in the sequence. I can reliably count half and even quarter seconds that way (eight or sixteenth notes in quarter-note = 60 bpm), but don't usually need such precision.
Making a test strip that way is EZPZ; just cover consecutive stripes on the strip with a card while counting. Example using my 25% test strip numbers above: Count 10 seconds, move card on the beginning of second 11, but start over with 1, count three seconds, move card at beginning of next second but starting over with 1, count 3 seconds, etc.
The sequence looks like: - 10 -- 3 -- 3 --- 4 -- 6 --- 6 -- 8 (and OFF).
That gets me my desired:
10 - 13 - 16 -
20 - 26 - 32 -
40 seconds, which I find a good range of exposure times and easy to remember.
Voilà, a progressively spaced test strip in approximate 25% (= ~1/3 stop) increments. No charts or calculators and minimal figuring of percentages.
I like simple tools. Why spend money on technology if it doesn't give you an advantage? Carlton Watkins prints aren't worse because he didn't have an f-stop timer
Best,
Doremus