Since you aren't concerned with any historical or period authenticity, where you place the rear sight is strictly up to you. Something to keep in mind is that the shorter the sight radius is, the quicker/easier you can align the sights when aiming. The flip side is that a longer sight radius makes the gun more accurate. I don't know where you are in your build, but you might want to install the front sight sight then place the barrel in the stock, sit the rear sight on the barrel and move it forward and back until you find the spot that suits you best, then install the sight there. Same process works for a rifle.
Don Richards