I'm not sure, but I think that extremely shallow tapers turned on a cnc are still 'digital' in that the X axis moves a bit, pauses while so and so much Z travel occurs, then another X pulse. There is simply no interpolated motion possible when the command is less than a tenth, or whatever your machine resolution is.
Apart from that, have you ruled out relevelling the machine to get a straight cut? .0004 in 8 inches is straight, AFAIC , but a little tweak on the levelling jack on one side of the tailstock end should dial it in straight.
I hope you were turning with coolant all over the part and the toolholder. Only .0002" is the real error!