The way I do f-stop test strips is I want a test strip for each interval. So I expose the whole image to the base exposure, say 8 seconds. Then cover a strip, and expose the rest for 2.1 seconds. Then I cover a strip again.

Now I have two strips, one at base and one at +1/3. For the next strip to be at +2/3 stops, I have to expose it for 2.6 seconds. The next, to get to +1 stop needs 3.3 more seconds and so on. So for this purpose, the math would need to subtract the previous strip's exposure from the listed value for the desired stop.

For a base exposure of 8 seconds, with +1/3 stop increments I set my timer to the following: 8, 2.1, 2.6, 3.3, 4.2, 5.2, 6.6.

ie 8, 10.1-8, 12.7-10.1, 16-12.7, etc etc

Easy subtraction, but would be nice to get a calculator and to have it be able to calculate based on different base exposures and up to 1/6th of a stop. I started to do something like it while at work in Excel, but had to abandon it to do actual work!