If you want each step to represent a stop of light you’d need to double the time for each step. Matching the time to f numbers will not work. The f number is the focal length divided by the diameter of the aperture. Each f number (for a full stop, like 5.6 to 8) represents an aperture with double the area. The key relationship is that a “stop of light” means twice as much light. Two light bulbs vs four, 10 mm2 vs 20 mm2, or 5 seconds vs 10 seconds. Of course doubling the time relies on the fact that you’re not dealing with reciprocity failure, which for cyanotype, I think you probably are, so really just going with a linear scale is probably as good as anything.

@@plateoshrimp9685 Going in steps of 1/2 stop gives you a finer gradation. If you are trying to pick the exposure time that gives the best print, you want fine gradations. By using half stops, you are linear on the density. Going linear on the exposure time gives and uneven spacing on density which does not allow as close a read on the quality of the image.

Additional exposure won't add any benefit to the result. Maximum density is maximum density. Think of it as the first step in a row of calibration techniques. Once you know your base exposure time, it's much easier to reproduce results, to prepare negatives' contrast for the desired print results beforehand etc.. Base exposure time helps you keeping all possible detail in the highlights as this would become dull and duller with any added exposure (less over-all contrast). To accomplish this level of calibration it's advisable to use controllable, artificial light sources, since sunny daylight is subject to change (think of clouds passing by, daytime etc.).