[Ian C]

With respect to post #10:

I find it easier to meter the projection through an empty carrier for maximum brightness to find the difference in stops. I use the aperture wide open for the brightest projection.

I first focus the smaller projection, remove the negative, replace the empty carrier, meter the center of the projection, and record the reading.

Then I replace the negative, resize the image, focus, remove the negative, replace the empty carrier, and meter as before. The difference in readings is Δf. Then the time factor is

2^Δf

The new exposure time is

(2^Δf)*original time