I have patterned a lot of shotguns and all full Chokes pattern #4 buck tighter than a mod choke...my luck of the draw so far. Shotgun throats and bore dia's are all over the map when it it comes to size. Wierd thing about patterning buckshot, one shot may pattern tight and the next is horrible, point of aim is usually off from where you are pointing also...shoot more than one pattern.
Realistically, you should pattern at 40 yards. Too often guys pattern at 20 or 30 yards and 10 yards really means a lot when it comes to patterns opening up, especially from 40-50 yards.
50 yards is a very long shot with a shotgun, unless you are shooting a 10ga with 52 #4 buck or 2 1/4 oz of copper plated BB's.
At 40 yards, I look for at a minimum of 85% of my pellets to be within a 30" circle with a standard full choke. With a Turkey choke, I will get 100% within a 20" circle at 40 yards.
A very good way to patten is to put up a large piece of 36" or 42" paper, then put a paper plate in the middle. The center of the paper plate is the kill zone on a coyote, results will be very clear. If you do not have large pieces of white paper, then tape 4 pieces of news paper together to form a large square, the put the paper plate in the middle. If you live in a rural area, an old wood pallet makes for a good pattern board backer.
GC's advise is right on.