First off, I usually use a faceplate and live center for that
operation, not two centers. It's easier to get the blank positioned
properly, but takes a little more time. Build yourself a giganto-spur
by sharpening three or four machine bolts to a point, and bolting them
to your faceplate to form a three- or four-prong spur. As they'd be
further apart, they'd just naturally force the flat parallel to the
faceplate.

Second, I think it's expected that you'd turn the blank by hand and
see if it's doing what you want before turning on the lathe. Some
adjustment might be needed. As you turn it, see where the edge of the
flat part (drive center side) pass by the toolrest; if they all pass
at the same spot it will be close to parallel.

Third, I often use a Dewalt powered plane to carve the blank a bit
after it's mounted, to get it more balanced. This is after the rough
carving-to-shape with a chainsaw. FYI: for reeves drive, it's easier
to hand-balance the blank if you set the lathe to the FASTEST speed.
That gives the spur a mechanical advantage over the motor, and thus
reduces "friction". Remember to slow it down again before spinning up
the blank for real. If you have step pulleys, just take the belt off
or de-tension it.