Minimum body taper depends entirely on the angle at which the cartridge feeds into the chamber. An axial (straight-in) feed accepts cylindrical cartridges (NO body taper). A cartridge that enters the chamber (at first) at some angle to the axis of the chamber must be tapered. So the minimum easy-feeding taper is a matter of how the cartridge enters the chamber.
<br>The practical answer is simple -- just be sure to have the case taper noticeably. The cylindrical-bodied necked case offers no real ballistics advantage over the tapered case, so you might as well use a tapered case.