I have a Vortex venom and a meadow creek mount. When trying to sight it in the up turret was maxed out and the red dot was not on the target. I called Meadow creek and was told to turn the turret up and down a couple times and it may fix the problem. I have noticed that there is a shim to put between the red dot and mount. How many of you have used either method and do they work? should I go ahead and buy the shim? This is on a 870 20 gauge.