A heading error of 0.25/hr at 480 knots equates to an azimuth error of about 2 nm/hr. Given a flight time of 6.5 hrs that could give an uncorrected error of 13 miles - well within the accuracy requirements for the H2S to update and acquire the target.
However the mission was a night mission with the availability of astro. The norm was a fix every 40 minutes but over that length of sortie it is likely they would have dropped to one fix per hour. Terminal errors, given a serviceable doppler and HRS would have been no more than 10 miles - about half the error of the dop/HRS alone.
Given a good autopilot and calm conditions at astro could have been better than that.