An approximation is presented for the projectile-bound nucleon scattering amplitude in terms of a "quasi-free" amplitude that minimizes corrections. This optimal choice permits factorization of the impulse term into a free on-shell amplitude and a form factor. The first nonvanishing correction terms are estimated.NUCLEAR REACTIONS, bound nucleon, scattering matrix, minimization of corrections, on-shell projectile-nucleon amplitude, factorization, higher order corrections.