If I understand your original question correctly, you will want to match both the dark and light ends of the scale, not just the dark one. The major difficulty in making copy prints (positive/negative/positive) is in controlling not the density but the contrast. It is very easy to get excessive contrast from printing paper, which is one reason why in-camera paper negatives are often pre-flashed.
Dealing with contrast isn't much of an issue. Contrast is easily controlled by either paper grades or contrast filters. As to the step wedge, all you have to worry about is printing the reciprocal of one step on the wedge. The wedge, whether 21 or 31 steps, is designed to record tone in 1/2 or 1/3 stop increments. Those measures are fixed. Choosing one step on the wedge and printing its reciprocal will render all other steps printed to their reciprocals equally. The only remaining issue is contrast as you described and that can be managed through the use of filters or paper grades.