The formula to convert stops to time is:
Originally Posted by blanconegro8c
T = 2 ^ stops
put in any value of stops you like. Typically you test with exposures that are uniformly-spaced in stops, e.g. 2.5, 2.75, 3.0, 3.25, 3.5, 3.75, etc. Convert each of those using the above formula into seconds to get your sequence of printing times.
If you want a nicely printed table, there are f/stop timing cheatsheets and spreadsheets you can download, or just enter the formula into Excel and print out a table of times against a table of stops. I recommend getting a copy of WBM2 as it has all the instruction you need for your printing; see also the FAQ in my signature for a very simplified intro to printing.
Typically when you do a test strip, you will test initially with 0.5-stop intervals. Once you get something ballpark-right, you can do another strip at maybe 0.1-stop intervals.
As a start, the default whole-stop aperture markings on a camera are half-stop values when used as enlarger times, i.e. 4, 5.6, 8, 11, 16, 22, etc. And the 1/3-stop sequence on an aperture is a 1/6-stop when used as times, i.e. 4, 4.5, 5, 5.6, 6.3, 7, 8, 9, 10, 11, 13, 14, 16, 18, 19, 22, etc. They are heavily rounded-off though for convenience but they are kind of close to the right progression.
Edit: ha. I should type faster