The best rule is NO choke or cylinder choke. You can do other stuff but you loose accuracy and gain danger from leading of the choke and shearing of the slug or sabot. You can eventually end up bulging your barrel or worse!!!!!!!!!!!!!!!!!
I have given up on all chokes myself, as from the patterning I've done with my guns -- the cylinder bore, short barrels, with modern shells, hold just as tight a pattern as longer barrels and chokes. This is what I've found with my Super 90, 11-87, 870 and Mossbergs. The modern shot cups, more perfect barrels and the better shot seems to be what makes for such improved patterning. As they say, your mileage may vary.
Take care,
Jody