How should one determine when to perform Ghusl if their menstrual period stops and starts towards the end, and they are not sure if it has fully ended? Is it permissible to wait a day or two before doing Ghusl to be sure?
You need to be sure that the menstruation cycle has ended and there is no blood at all, then perform Ghusl of Haydh. If you doubt that the menstruation might not finished yet, you need to wait and check till you become sure that menstruation has really ended.
Minimum period of menstruation is three days continuously and maximum period of menstruation is ten continuous days. Bleeding after ten days is not menstruation but Estihadha during which SALAH is obligatory with rules of Wudhu or Ghusl.
Wassalam.