drummer,
If you're really doing it right, your anchor point should not change significantly although your draw length will probably increase. If your string hand has moved back that much I think you're not rotating your drawing elbow/shoulder as much as you think but rather pulling more straight back. Also be sure that you are extending your bow shoulder low and toward the target. If your bow shoulder is colapsing back toward your spine, that will move your anchor hand back too.
Once I reach full draw, I think of the final transfer to my back as sticking my chest out a little to get it between my shoulders.