Quote Originally Posted by johnnywalker
Is there any reason why you can't simply calculate it
using the square inches of the print? If a 4x5 (20 sq. in)
print takes 20 seconds, then an 8x10 (80 sq in) will take
80/20 = 4 times as long at the same f stop.
Perhaps this matter has already been mentioned. As
the negative's distance from the baseboard increases
and the focus is adjusted by bringing the lens closer
to the negative, the speed of the lens increases.
Just as one would expect from a camera lens.
So, with your example the time might be
3.5 times as long.

I also recommend the EM-10. If calibrated it can
be used as a densitometer. Same exposures are
obtained by adjusting the F stop. Dan