I'll assume here that you are interested in using slugs, and have an adjustable sight of some kind.
If no 28ga sight arbor shows up, I'd just try it out at short range, maybe 25 yards. Get it just above the bull there, and that ought to get it on the paper for 100 yard sighting, with little if any further adjustment required.
Careful use of a dental mirror will allow you to get the bore lined up with the target, if you've got a pump or an autoloader there. You'll want shooting bags or a sled for this. Then, without moving the gun, take a look to see where the sight is looking, make an adjustment, and then compare the two again.
You can get a dental mirror at most drug stores. They are handy for all kinds if stuff, a good investment.
Edited by charlesb (03/28/16 08:00 AM)
Kind regards, charlesb