Some approximations of the Haydock-Kingham method to determine the hydrogen field ionization rate are discussed and the necessity of corrections is demonstrated. It is also shown that the matching point between the ground state and JWKB functions is field dependent, even when using an atomic function close to the nucleus as in the Haydock-Kingham method. The possibility of adopting a Stark function close to the nucleus, instead of the atomic one, is included.