Thank you both. I did measure it and based on that I ordered a 5L390, 39 inches. That was also verified by the length formula. Unfortunately I did take the counter shaft into account. It is not a v but a slightly dome surface, the 5L390 belt is too short. If I extrapolate the counter shaft to v dimensions then the equation estimates 40 inches. I have ordered a 5L400. I will update and confirm the posting.