I have never found a pulley separation calculator that got it right. I draw the two PCD's at around the separation I want, add two tangents to connect them as the belt would, trim the circles to those tangents, list the two arcs and one tangent then tally their lengths. Undo until I am back at two circles, tweak the separation and start again until I get it near as makes no difference. If you mount motor and shaft bearing in the same piece of metal you don't actually need any adjustment if your machine is accurate.

Make sure none of the pulley bolts coincide with the belt and if you are really canny you add a fixing for the plastic conduit that protects the motor leads, because that is never in quite the right place if you forget it now