As above fit matters alot. In fact, while most shotgunners are worried about choke they should be devoting most if not all of their worries towards fit. That said, it's best to involve a stockmaker that knows what he is doing. You said "how can I determine the correct fit for me?" Tape a flashlight to the barrel {or a laser} and carefully align it so that when you look flat down the rib the center of the light beam is centered to the bead. Look at a spot you want to hit, close your eyes and keep your head up, throw the gun up and see if the light beam is anywhere near it. Try this with your eyes open and closed. If it's consistently to one side you need cast, if it's consistently up or down you need more or less drop. Have someone else watch you to see if you pull the stock back into your shoulder...if so it is too short. This plagues many shooters, just about everyone is shooting way too short of length of pull. You should be pushing the gun forward towards the target. If you are pulling back with the gun as you point it this will cause you to shoot behind a crossing target. I am 5'8" and shoot a 15 1/8" LOP. Most factory guns are around 14 1/2", most shotgunners have longer arms than me. All this aside, yes you can "overcome" a gun that dont fit and most american shooters dont even know or realize they are way past that point. You "overcome" a poorly fitting shotgun by aiming it instead of pointing it. I suppose this is why so many guys worry as much about the choke... you can overcome a poorly fitting gun but there isn't much you can do about the wrong choke. One important point about doin' it yerself...most guys dont even realize they are aiming instead of pointing and without someone else to help them they still wont get the full benefits of a well fitting gun. Throwing all the above aside one more time...this gunfitting thing has more application in wingshooting or fast moving targets. To hit a coyote stopped at your decoy at 70 yards with a load of Hevi-shot you just aim.
