There is a lot more to it than that, but basically it boils down to the clearance between the needle OD and the nozzle ID at a given slide position that determines available fuel. The straight diameter affects this off idle before the taper comes into play by reaching the nozzle. The slide cutaway affects airflow at a given throttle opening (small openings). Ideally all this is balanced and if it can be power will be smooth and clean. If not, you can be rich in one area and lean in another. The stock needles are actually triple tapers (angle changes three times over length). The more narrow tip means that the overall fuel flow can be more, and will generally require a leaner main jet than a single taper needle.
From my experience, if the slide is wrong its difficult to dial in settings with any needle. Jetting three different 250s, the #7 slide is much eaiser to work with than the #6 at 0 - 1000'. For a 300 I'd try a #7 vs #8.
