If you are like most boat owners, you will be doing this while out of the water. This means no guess work. You simply remove the old shaft, then take it to a prop shop that has machining capabilities and ask them to make a new one. It took 3 days for the shop in Everett to fabricate a new shaft. Match the taper to my prop. Then fit and face a coupler to the transmission end of the shaft so that the transmission can be attached and aligned to the new shaft.
That way all fits are accurate and no guess work about... Is the other boat "exactly the same" as my boat.