Greetings Tom and welcome to EM,
Thanks for posting that -- is this the chart you are referring to?
If the length of the boattail is less than 1/10 of the length of the car, then the maximum/ideal angle is ~14 degrees. If the length of the tail is 1/4 the length of the car, then the angle is ~17-18 degrees. And if the boattail is 1/3 the length of the car, then the maximum/ideal is ~19-20 degrees; and it yields a drop of 0.07+ in the Cd -- if I'm reading it correctly.
So, on my proposal model, the maximum angle can be ~17-18 degrees, since the tail is about 1/4 of the overall length after it has been added.
This chart looks like it might have come from Hucho -- can someone confirm?