I don't have any experience mounting optics on shotguns, but I will recommend you don't sight in using a lead sled or any rest that clamps the gun in. Just bags. Turkey loads generate a lot of umph, and I would be afraid of damaging the optics if the shooter doesn't absorb some of that recoil.
Sight in as you would a rifle, using the center of your pattern as point of impact. I'd zero at 40 yards and check at 20 and 30 to verify where your hitting at those distances. If you plan on shooting further, test at max. You know your limits, or should. I assumed it was a turkey gun, given it's a red dot. If slug gun, distance will change, but the rest of the info holds true.
If you're set on a saddle mount, B-Square makes a good one.
To sight in with shotshells, get a roll of 30" (minimum) wide paper. You can get it at Walmart. Grab a handful of low-brass. Shoot off a good rest at 25-30 yards...…. 3 shots on 3 different sheets of paper. Mark the center of your maximum density on each sheet. Measure how far left or right of your point of aim and how high or low. Average the 3 measurements and move the sight accordingly. Then move back to 40 yards and, using whatever shell you plan to shoot, shoot 3 more clean sheets. Measure and average again. Move the sight. Shoot again to confirm you are where you want to be.