Use the diameter of the ball as your projectile length as well as your diameter. In the bullet metrics (select icon of bullet with calipers across it) enter that length, enter the ball weight. At the bottom check the box on the left for boat tail bullet. Give the boat tail small diameter as 0.618 times the ball diameter. Give the the large taper diameter as the ball diameter. Give the small taper diameter as half the ball diameter. You should end up with the program saying you have a 20.9° tapered boattail.
What the above entries do is create a truncated cone in place of the back half of the sphere that has the same volume as half the sphere. This should allow powder volume to be correctly calculated.
Note that your exterior ballistics will be a bit of a pain to calculate. Round balls have squirrely drag functions that vary some with diameter. About the closest you'll come is to use QuickTARGET Unlimited software that came with QuickLOAD. Import the QuickLOAD data to it and switch the drag function to the 9/16" steel ball drag function.
Next, calculate the sectional density of the ball you are using (weight in pounds over square of diameter) and divide it by 0.082. The new number becomes the ballistic coefficient you use with the 9/16" round ball drag function for trajectory. It won't be right for any other drag function, like the standard G1 drag function (which doesn't apply well to a sphere anyway), but will do about what you can with a ball in a rifled barrel.
CORRECTIONS: I put in a radius rather than a diameter. I have now corrected it, above.
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