I used the A.R.M.S. #74 Spacer 1/2", this allows a lower 1/3 co-witness for my SightMark Red Dot optic. For me this was a natural position for my AR (where my cheek and stock fit placement was).

But everyone is different and you have to find your position. Leaving the optics directly on the rail places the rifle to tight and high to my face. Although rumor has it that it is always better to have the optics as low and close to the barrel, but maybe more so with a magnification scope vs. a non-magnification Red Dot. Good luck.