Take a ride to the Lowe's and spend a few bucks on a roll of contractors paper. Its 36x36" and $12 will buy you about 150' of it... more than you'll ever need. You want to make sure you capture as much of your pattern as possible.

While you are there... get a 100' measuring tape... and make sure you are 40 yards away.... not crucial... but I've seen some pretty wild guesses in my day. I think your set up will be fine.... you just want to make sure that the gun is shooting Point of Aim to Point of Impact (read: straight).