In subsonic flow, the transition between the cylindrical body tube and the boat tail needs to gradual to reduce flow separation from the surface for low drag. The longer the transition is, the better. Therefore, a tangent ogival boat tail would be better than other conventional shapes such as a conical boat tail. The boat tail needs to be held to a closing angle of not more than about 10 degrees, or else flow might separate over the surface of the boat tail. The closing angle can be altered by changing the length of the boat tail.
However, there are two limitations that need to be considered. First, due to the curved transition, the boat tail might interfere with the motor mount. There should be space between the boat tail and the widest end of the nozzle. Second, the boat tail should not overhang the nozzle by more than one body tube diameter because otherwise the Krushnic effect will be triggered and it will kill part of the thrust.