A 32-year-old Rh-negative multigravida (2 previous vaginal deliveries) presents at 30 weeks of gestation with vaginal bleeding. Ultrasound confirms a low-lying placenta with no evidence of abruption. Speculum examination shows cervical bleeding. Her husband is confirmed Rh-positive. Kleihauer-Betke test shows 2.5 mL of fetal red cells in maternal circulation. She received anti-D 500 IU at 28 weeks. What is the appropriate anti-D dose to be administered now?