Pattern your gun first... both for Point of Aim (POA) and Point of Impact (POI).
I do both at 16 yards, generally with a Full or Imp/Mod choke.
POA is "static" ... basically "bench-rest" your shotgun as you would a rifle ... you're trying to see if the shogun actually shoots "straight" to where you are carefull "aiming" it - rib dead flatand bead(s) centered on a small 2 or 3 inch "aimpoint". POI is more "dynamic" - done standing up, shooting as you would in the field or on your station. Raise the gun in one smooth motion
( say, as if it were a lazy straight-away and a very gently rising target ) just the insant you are "on" ( no delay, it has to be virtually "instinctive" ) pull the trigger. Above all, don't "aim" just shoulder the gun and shoot - like you normally would - any attempt to "aim" is counter-productive. Many state every inch of POI error requires 1/16" correction to the comb, either up or down - or cast-on/off to correct. For a right handed shooter, shooting consistantly high and left for example, would reguire a little more cast off and the comb to be lowered ( provided LOP and Pitch were already correct).
Length of pull and pitch can vary things a little, but all contrbute to the mechanics of a smooth, consistant, comfortable and repeatable gun mount, which in turn contributes to having "the gun shoot where you are looking".
Changes should be approached rather cautiously and on the conservative side ( you can't put wood back on where its been taken off !) An experianced gunfitter/gunsmith is a valuable asset.
Pattern for choke/load after the gun "fits" and is "shooting where you are looking" ... for 16 yard shooting, 32-34 yards is about right, for the back fence, 44-46 yards. Pick one distance for each and stick with it.
45 yards works.