Obviously a very old thread, brought back to life. There's a method published in Way Beyond Monochrome for quickly determining the magnification. Cutting two V-notches into the edge of the negative holder, one inch apart, will project at the edge of the image on easel. Measuring the distance between the projected notches instantly gives the mag factor. The idea of defacing my negative carrier does not really appeal to me, so I've been using a transparent ruler on top of the negative carrier, then measuring its projected image to find the mag. That's certainly accurate enough IMO. Changing the mag requires a correction to the exposure time; the new exposure time is calculated by this formula: T2 = T1 [ (M2+1) / (M1+1) ]^2 EDIT: This formula provides the exposure change but, as conanw noted, there'll likely be a contrast change as well, necessitating a new test strip. For split printing, however, applying the exposure correction to the highlight exposure time may avoid the need for a test strip.