If you can manage it, don't stop down so far as it will really reduce the sharpness of the image. Your enlarger may have neutral-density built-in, if it is the diffusion type, or use a piece of ND filter in the filter tray. Small enlargements are sometimes tricky with a bright enlarger as the time is short, but a tiny aperture (and hence a bit of diffraction) might not be the best solution. To save fiddling around with step wedges use your contact-sheet as a sort of enlarging-meter. Do a search on in here for contact-prints and maximum-black.