Mario,

If the final aspect ratio is not 3:2, there is no one answer. You can go by width, height, or diagonal. Take the ratio of the respective measurements and multiply the focal length. So if the final image has dimensions, say, 120mm by 74mm, you COULD say, using diagonals, that the effective FF focal length would be 100 * Sqrt[24^2+36^2] / Sqrt[120^2+74^2] = 31mm. If you used the long edge, you'd get 100 * 36 / 120 = 30mm. By the short edge, 100 * 24 / 74 = 32mm (all answers rounded to the nearest millimeter).

For a squarer or wider image, the three answers would be more different from each other. For a 3:2 image, the three numbers would all agree.

Best,

Matt