Try the enlarging correction ruler at the Darkroom Automation support files web site.

You measure the size of the original image and the size of the new image. The difference is the exposure correction in stops.

Example:

Original image measures 5.2
New image measures 3.7
Difference is 1.5 stops: open the lens aperture by 1 1/2 stops or multiply the time by 2.8

A table of stops<->seconds and a stops dial for GraLab and Time-O-Lite timers are available on the DA web site support section.

Or you can use an enlarging meter. The already mentioned Ilford EM-10 requires you adjust the lens aperture to null the meter at the new magnification. The Darkroom Automation Precision Enlarging Meter lets you adjust the time, the aperture or both to correct for the new print size. An f-Stop Timer is handy for correcting exposure in stops.