Using an f-stop timer without an enlarging meter to provide exposure compensation can be easy ... if one measures distance in stops. With the right ruler one need only measure the lens height or image size before and after the magnification change and subtract the two readings - the result is the required correction in stops. A pdf file for such a ruler is provided at: http://www.darkroomautomation.com/support/stopsruler.pdf As an f-stop timer works in logarithms (stops being logarithms base 2) the calculation of the correction becomes simplified if the distance is also measured in stops: division turns into subtraction and squaring turns into doubling. A logarithmic ruler marked in 2*log2(distance) reads the correction factor directly. If great accuracy is needed then an exposure meter should be used. Different enlargers require different corrections for the non-square-law light fall off from the condenser or diffuser to the lens. This correction is only critical at small magnifications where the lens-light source distance changes greatly.