Let's go through that point by point:
If you say that you built your own turntable, you used a KUKA motor and built your mechanics "around it", right? I.e. the control is KUKA standard?
As the next step you would calibrate your turntable, as a result you will get the XYZABC values that signify how the turntable is located in relation to the robot's base. So if X is 1500, the turntable would be 1.5m in front of the robot. Enter those values in the dropdown menu that you get from right-clicking the turntable component in PRC.
Finally you would calibrate an Offset system. That is the same as a Base system, but its zero-point is the origin of your turntable. So the values will be rather small, in the hundreds of millimeters probably. Enter the XYZABC values as a base in KUKA|prc.
Before you proceed with KUKA|prc, activate a calibrated tool and your offset base. Go into e.g. base movement mode and move the E1. The tip of the tool should stay at exactly the same position in relation to the turntable, it rotates with it. If it wobbles around, your calibration isn't very accurate. Or maybe the gear ratio is incorrect.
The dynamic value by vector works OK for some simple cases, I usually now write my own small logic to set the external axis values. You can directly set E1-E4 by right-clicking a movement component and clicking the appropriate option.
Hope that helps!