To make it easy on ya', get the gun securely rested, so it won't move easily.
Use cheap low brass as 777 said. I'd use it at 15, 30 and 40 yds.
Take your shot at 15 yds. Keep the gun in the position you shot from (don't move the gun). Look through the red dot/reflex sight and turn the adjustment dials until the dot lines up with the point of impact on your target. Shoot again to check point of impact.
Move target to 30 yds and repeat. Same at 40 yds. Don't worry if you think you're dialed in at 15 or 30 and find that you're off at 40. Find center of pattern and adjust as you did at 15 yds.
Now shoot your turkey load at 30 yds. You should be dead on or close. Make your adjustments if you're not.
Now check with turkey loads at 40 and adjust if necessary.
Last, check at 15 with turkey loads so you'll know what that looks like.
It seems like a little bit of a pain but it's actually pretty easy if you go about it methodically. That and you won't be sitting there slack jawed wondering how you could have missed that chip shot at that gobbler you had dead to rights...