John,
As long as you have the plate centred up on the RT, there is no need to centre up the RT to the quill.
Just move the RT to a position where you want the holes to be in the plate, they usually give you the PCD with the chuck, if not, just measure the back of the chuck from the centre to the centre of the hole, mark up one hole centre on the plate and drill a hole, then making sure your plate doesn't move, if it is four holes, rotate it by 90 degs, and drill next hole, 120 degs if it is a 3 hole version.
That goes for anything you want to drill around a circle that is centred on the central position of the RT, whether two holes or twenty, as long as you have the first one marked on the job so that you can pick up on it, there is no need to centre up the RT. Drill the first, turn the correct amount of degrees, and everything will be spot on.
John
NB - make the holes in the plate say 1/2 mm larger, it is the accuracy of the backplate spigot that fits into the chuck that keeps it central, the bolts are only there to hold the chuck in place. By having the holes slightly larger, that ensures the bolts don't interfere with the chuck sitting on it's spigot correctly.