HH, in my experience, the "conspicuity curve" flattens out after you get to the left edge of the bike lane. From there to the center of the lane, there difference in "conspicuity" is neglegible. In other words, I agree with your a-d, but the curve is non-linear and there is practically no difference between riding towards the left side of the bike lane and in the center of the lane. The curve drops off rapidly as you move further right, and is extremely bad for a separated side path, where the curve drops with a discontinuity.