def calculate_utilization(billable_hours, target_hours):
    """Calculates utilization as hours billed divided by target hours"""
    if target_hours == 0:
        return "Non-billable"
    if target_hours is None:
        return 'No hours submitted.'
    if not billable_hours:
        return '0.00%'
    return '{:.3}%'.format((billable_hours / target_hours) * 100)