If you play with adjusting the compounding, note that there is both a high speed and low speed compounding adjustment. On each pole piece, there is either a high speed fixed and a low speed adjustable winding, or a low speed fixed and a high speed adjustable winding. Also, the spiral shunt will let you get to less than 10fpm difference. Also, shimming the interpoles in toward the armature will reduce the residual voltage. Use brass shims, not steel ones.