Start by adjusting the red dot to the tip of the front sight post while looking through the rear sight aperture. You can do that at home. When you get to the range go from there using any zero you want... same zero as irons or whatever. I zero my irons and red dot both at 50yds. I use a lower 1/3 riser. No matter if you use a lower 1/3 or absolute riser, if you use the same zero the red dot will still be on the tip of the from sight post when looking through the rear sight aperture. The only difference is with the lower 1/3 the irons and dot will be in the lower part of the tube. There is no right/wrong zero for lower 1/3 or absolute.
Looking through the rear sight aperture.... this would be an illustration of a rifle with irons and red dot using the same zero. Lower 1/3.
And this would be an illustration of the same rifle, same irons, same red dot, same zero, same lower 1/3 riser, but the shooter looking over the irons through the center of the tube.
This would be an illustration of the same rifle, same irons, same red dot but with an absolute riser and looking though the rear sight aperture.