The best sized choke would depend wholly upon which cartridges you had sold the shooter, in the first place. If they were fibre wads the choke may need to be a different size, to that required by plastic wads. Can I suggest that you obtain sufficient cartridges of both plastic and fibre wadding, a selection of various chokes suitable for your gun, perhaps even by different manufacturers. Take a minimum of 500 of each cartridge type, find a pattern plate where you can spend the next few months and carefully pattern each cartridge and choke configuration, making suitable notes as to the number of pellets in a 30" circular hoop placed in front of the aforementioned pattern plate. Do not forget to count each pellet and make notes accordingly, perhaps even taking pictures. From these tests you should eventually be able to assess for yourself which cartridge and choke combination you would need to provide for the best results for your customers. Perhaps then, you could resurrect this thread and enlighten the readership, all of whom I feel sure will be waiting with bated breath, of your findings and even display your photographic skills.
Of course in the meantime the said readership, myself included, will simply get on and enjoy our shooting, oblivious as to whether our choice of cartridge and choke combination is correct for the discipline we are shooting at the time. :fie: