To convert from time to f-stops try http://www.darkroomautomation.com/su...stopstable.pdf

You can use the 'seconds' column as the correction factor from the above equation (M + 1)/(m + 1). Say the correction is 1.75, then from the table the appropriate stops correction is 0.8 stops (opposite 1.74 'seconds').

Since enlarging lenses are hard to adjust accurately except at their click stops, and some are none too accurate even then, time correction provides a more accurate method. For large changes a combination of lens aperture and time may be convenient. Say the required change is +2.7 stops: open the lens 1 stop; then apply the remaining 1.7 stops by time correction - multiply the time by 3.25.

Of course, with an enlarging meter and f-stop timer the exercise becomes trivial.

A budget alternative is to use an Ilford EM-10 comparator and do all the adjustment with the lens aperture.