The truth is, unless the pen blank is really close in thickness to the pens finished diameter, the hole doesn’t have to be perfectly centered. Once you turn it on the lathe, it’ll work itself out. That said, using pen jaws on a chuck (as shown by Dave) does make it pretty easy to keep everything centered and square. 

Ryan/// ~sigh~ I blew up another bowl. Moke told me "I made the inside bigger than the outside".