Help Desk Staff to User Ratio

Good Morning,
I am a Help Desk Manager for a law firm in New York City and was just wondering what a realistic ratio should be for help desk staff to user. I realize that every company is different but currently I have three techs and we have over 400 users. If I count myself that is approximately 1 to 100. Is that right? Is there a rule of thumb one usually goes by?
Any information would be appreciated. Thanks.