I just cut one 3 times and it was still too short! Seriously, I had to lengthen the shaft on my sprayer to reach the pto on a different tractor. Here is what I would do when buying a new pto driven implement.
1. Check the owner's manual. Should explain how to check the length and cut if needed.
2. If that fails, ask the dealer. Maybe they will do it as part of the deal.
Let the board know how you made out. If the above fails, there are people here that can guide you through it. But try for the manufacturer's specs first, for they sometimes have minimum overlap specs and/or a certain position (up/down) that the machine needs to be in while taking the measurements. I did a quick & generic setup when I lengthened my sprayer shaft, but I didn't worry about minimum overlap because it requires minimal hp. A tiller will need more ponies.