Normal Post-Void Residual Volume Calculation

A 68-year-old female patient is admitted to the hospital due to urinary retention. After assessment, it is determined that she has not voided in over 12 hours. The nurse assesses the abdomen and finds it to be distended. The physician orders a bladder scan which reveals a post-void residual volume of 500 mL. The nurse understands that normal post-void residual volume should be less than ____ mL. This information is critical in determining further interventions for managing the patient's urinary elimination issues.