Using the Star sizing unit, size your bullets nose first, and you'll get much more concentricity than base first sizing, as the ogive of the bullet will guide the bullet in straight to the leade of the sizing die, and if using a closely fitting diameter flat punch, simply place the gas check on the heel of the bullet, and the flat punch will positively seat the check square and true with the axix of the bullet, and crimp it on as the puch pushes the bullet through the die.
By the way, don't believe that garbage about one flat punch diameter working from .35 through .475 caliber! That is pure hogwash! If you want the checks to seat surely and squarely, use a flat puch that is no more than .003" under the diameter of your sizing die! In this manner the checks wont pull back from the heel of the bullet when being pushed through the sizing die nose first!
Hope this helps!