Assuming you have the f/4 80mm Rodagon, it's a 6-element lens with 4 groups - one of the better "standard" designs, optimized for 2x-10x (6x optimum) magnification (manufacturer specs). You might get improved results with one of the APO enlarging lenses. The 80mm f4 APO Rodagon, for example, has a (stated) magnification range of 2x-15x (10x optimum).

As to the sharpest f-stop, that's usually 2 or 3 stops down from wide open, but running your own tests will show what your optimum f-stop is.