Take the 3 or 4 days on the slip. All prop tapers and keyways are machined individually and a variation of 1/4 degree or less could see you buying
another prop or the added expense of remachining & fitting spacers etc. Also, make sure after machining that the prop is lapped onto the shaft properly and check for key binding (ie, the prop bottoming out on the key and not the taper). To check for this, fit the prop without the key installed and scribe a mark on the shaft at the forard edge of the prop boss. Remove the prop and refit
with the key installed and check that the forard edge of the boss still lines up with the scribe mark. The prop does not need to be cranked up tight for this check, I would hand tighten the nut and then give about 60 degree turn with a wrench.