If you've got a soft negative (which it seems you have) it is sometimes easier to establish your G.5 exposure first. Determine the exposure which gives the required density in the shadows and then dial in increasing G.00 'til your highlights are where you want 'em.
I would tend to disagree with this. Remember you are taking a white piece of paper and making it shades of gray to black, or thereabouts. Once you have set your dark's you cannot make them lighter, rather set your whites and darken them as needed. You cover white with black, you cannot cover black with white.