I would make sure your Stevens is a full choke then go out and pattern it with the cheapest bulk birdshot around. Chances are that's what they will probably be using.
Most of the shoots around here are either 30yds or 50yrds. I would start by patterning at 30yds.
One great and cheap ways I've found to do this is by getting a cheap or left over roll of christmas paper. Stretch it out white side showing along a fence or in between trees, staple and put a few x's on it with a black marker a couple of feet apart. Shoot the x's, check, adjust, then just roll out another section for round two.
I only takes one pellet in the right spot to win but if you don't know where to aim to get the most dense part of the pattern in the card your just cheating yourself. Good luck.