If it were that critical, you wouldn't see countless pros and others compensating for a too-small frame by cranking their setback to the max. That being said, assuming your frame is close to the correct size (and not too big), the correct setback relates more to your fore/aft balance than any KOPS relationship. The KOPS relationship approximates the position in which you will not experience too much weight on your hands, and one in which taking your hands off the bars will not result in the need for a drastic rearward flinging of your CG to enable the move. The smaller the frame, the easier this move becomes. This is an over-simplification, of course, but the point I'm trying to make is that the plumb-bob is not necessary. Other aspects of the fit are more practical to evaluate, and sometimes the frame choice dictates some of the fit aspects and makes analysis of someone else's position difficult.
Smaller frames generally have steeper seat tube angles, so you have to run more setback to achieve the required BB-saddle distance. this is why you commonly see pros on 25mm+ setback seatposts with their saddles pushed all the way back. Also, the right amount of setback doesn't have to result in satisfying KOPS; for instance, I have long femurs and I am well behind where KOPS would put me.