You are partially correct. There are two regulations that provide guidance on the subject. These are AR 600-9 and AR 350-1.
AR 600-9 para 3-4 states:
3-4 a. Weigh-ins and body fat assessments will be conducted in accordance with appendix B. All Soldiers will be weighed every 6 months, at a minimum.
3-4b. In order to ensure the ABCP does not interfere with Soldier performance on the APFT, commanders and supervisors are encouraged to allow a minimum of 7 days between APFT and weigh-in, if feasible. Some Soldiers that are close to exceeding the screening weight may attempt to lose weight quickly in the days leading up to a weigh-in. This practice may result in the Soldier being unable to perform his or her best on the APFT, if the two events are scheduled close together.
AR 350-1 Appendix G-9g(3) states:
Individuals will be weighed when they take record APFTs or at least every six months in accordance with AR 600–9.
So the options are: once every six months, or in conjunction with the APFT, but it is recommended that the Soldier get 7 days between the APFT and the weigh-in (if feasible). Therefore in my opinion there is sufficient lattitude to allow the Soldier to weigh-in at a later time.