The above equations are correct, but it must be mentioned that the paper-to-lens distance must be used (to be precise, it is the distance from the paper to the front nodal point of the lens). Some use the paper-to-negative distance, which returns wrong results.

As an alternative to the lens-to-easel distance, you can also work with the negative magnification. This method is very precise and often easier to measure. You can determine the actual negative magnification by having two notches in the negative carrier (I added mine with a file), exactly one inch apart, and measuring the distance between the notches on the baseboard. When changing the magnification from m1 to m2, the new exposure time (t2) is calculates as:

t2 = ((m2+1)^2/(m1+1)^2)*t1