Here is a table I use to guide me as I progressively cover more of the print. Each strip ends up being exposed for a total time equal to the interval indicated, plus the sum of the times for all previous (now covered) strips.

It gives me individual test strips that are exposed for the following series of total times: 4, 6, 8, 11, 16, 22, 32, 45 and 64 seconds.

It has the advantage of requiring that the enlarger go on and off just once (at the beginning and end).