That's subjective. IIRC the 0.052 is the closest to the width of the black, but a lot of people prefer a wider post. If you're using a 6 O'Clock, I suspect a 0.052 or 0.062 is preferable. I use a center hold w/ a 0.072 post. This is noticeably wider than the black when it's bisected - so it's easier for me to center the bullseye accurately.
Good thinking, the middle is always the middle!