There was a thread about this a while back. Now that this new thread is a 'sticky' it needs to come to the correct conclusion.
The equation I use is:
new_time = old_time x (new_M +1)^2 / (old_M+1)^2
where M = new magnification (print/neg) and m = old magnification (print/neg)
The exposure time factor would be:
Factor = (M + 1)^2 / (m + 1)^2
I wonder if Nicholas could supply the magnification in the above examples. I would like to see how this equation (converted from 'factor' to stops) compares to the metered values measured by Nicholas.