How much clearance will the new crankset have between the inner ring and the chain stay is another factor besides chain line (as in you need to have a few MMs of gap). So one might mount the new crankset on the old BB and see what chainline and clearance exists then from that choose a new BB that corrects these dimensions as needed.

As to why some cranksets have two BB spindle length suggestions- it's often for how the ft der works. A seat tube which is larger will position the der cage further out from the frame's centerline. So to not have the der cage no be able to rub when in the lowest gear combo the crankset needs to be slightly further out also. If the ST were of a smaller diameter then the cage inward range limit will be closer to the frame's centerline and a shorter spindle can be used.

So you see there are a few factors that are almost only "fit it and see" aspects of best function. Andy.