Depends on how you are bending. Thanks for this thread by the way. My posture sucks in every day life, so when on my road bike I try to bend from the hips...arching my lower back. I haven't found the forearm length notion very effective, but I do find the hidden front hub a decent indication of reach. For me. On my current set up, if I "slouch", the front hub is hidden by the handlebars. If if bend from the hips, the hub is in front...slightly. I obsess about this. the reason I obsess is because I get numb right had fingers, and right had toes. Left side is fine. Lower back, per your post, is not really an issue. I expect a bit of discomfort there regardless of if bad posture or bending properly. For me, back discomfort is not an issue. Right hand numbness bothers me. Perhaps related.