On my 23, I tighten the uppers first, using a long tape measure pulled up with the main halyard to get the length from the top of the mast to each chain plate even - that is, to center the mast. Once these are the proper tightness and centered I do the lowers. I don't have a gauge, so I just tighten them by feel, so I can pull the stay maybe 1/2 inch - who knows if I am right, though? Then I do the lowers by feel, and sighting up the main mast groove to make sure it is straight - you can pull the center of the mast sideways with the lowers so it doesn't curve.
I can't recall how mine pins at the bottom, but assuming you have two attachment points, one for the stay and one for the jib tack if you didn't have the furler, try using the one that is a further distance from the masthead if not already using it - that would allow you to tighten the stay more. On mine, I can twist the drum assembly until it gets pretty tight and am not nearly all the way in the threads.