Errrm....Errata....The formula should be (M + 1)^2/(m + 1)^2....

Changing from 4x to 5x requires a 1.44 exposure correction, about 1/2 a stop. As a crude check on this point a 4x5 print is 20 square inches, a 5x6 print is 30 square inches, so a correction in the neighborhood of 1.5 is reasonable.