I test in 1/3-stops as well. The benefit of this method is that every strip has the same apparent visual increase in exposure effect because it is geometric in nature. Conversely, a constant increase in time by some fixed interval (e.g., 4 seconds) produces less and less exposure effect in each subsequent strip as the cumulative exposure increases. For example, going from 4 seconds to 8 seconds is 100% increase in exposure while going from 36 to 40 seconds is only about 10%.

While you can buy an f-stop timer to do a geometric sequence automatically, it isn't too hard to figure out a working sequence and set a timer manually. The sequence I use is:

start @ 8-secondsat some intermediate f-stop (e.g., f/11)

add2seconds to get 10 cumulative which is ~1/3-stop more

add2.5seconds to get 12.5 cumulative which is ~1/3-stop more

add3.5seconds to get 16 cumulative which is ~1/3-stop more

add4seconds to get 20 cumulative which is ~1/3-stop more

add5seconds to get 25 cumulative which is ~1/3-stop more

add7seconds to get 32 cumulative which is ~1/3-stop more

add8seconds to get 40 cumulative which is ~1/3-stop more

add10seconds to get 50 cumulative which is ~1/3-stop more

add14seconds to get 64 cumulative which is ~1/3-stop more

The full sequence above will give a series of strips that cover a 3-stop range in 1/3-stop intervals that appear equally distinct from one another. You will almost certainly hit the proper exposure somewhere in there if you are printing a decent negative. You could actually start at any initial time in the cumulative sequence (and end anywhere) as long as the next exposure follows the other sequence. IOW, start at 10 seconds then add 2.5 seconds, then 3.5, 4, 5, 7, 8, 10, 14. Every third number in the sequence doubles so the next few exposures to add would be 8 x 2 =16, 10 x 2 =20, 14 x 2 =28, etc. So, it is easy to generate the entire sequence from any starting point if you can just remember the next three sequential additions (e.g., start at 16 and then add 4, then 5, then 7).

FWIW, shifting the decimal point in the ISO sequence for films (i.e., 80, 100, 125, 160, 200, 250, 320, 400, 500, 640...,) produces the cumulative exposure sequence above. Remembering that ISO sequence, I can quickly determine the cumulative time needed to expose the print. For example, if the 5th stripe looks good I think 80, 100, 125, 160, 200, so I know that stripe equates to 20 seconds for the print.

I keep a two-column table of these numbers on the wall next to the enlarger for quick reference. The first column is the exposure addition needed and the second column is the resultant cumulative exposure. That makes it extremely easy to generate or interpret a sequence of test exposures.

Hope that all makes sense. It is much, much easier to do than to explain.