f-stop print size exposure adjustment ruler on the Darkroom Automation web site.
It works with "f-stop timing", which is a good technique to start using early on in your darkroom work. To get started with f-stop timing and your present timer, an f-stop timing dial for a GraLab timer and f-stop timing table for digital timers are provided the Darkroom Automation web site. A true f-stop timer is the best way to control enlarger exposure.
The other way to compensate for changes is to use an enlarging meter. The Darkroom Automation Enlarging Meter will read the exposure change in stops. You can also use an Ilford EM-10 exposure comparator which will let you use the lens aperture to adjust for changes in print size.