Staffing Ratios Anyone?

Does anyone have an industry respected source for desktop support technician staffing ratios. I currently have 2 technicians supporting 550 users (over half are engineers) giving me a 275:1 ratio. Some years back at a Gartner Group conference I thought the ideal ratio was 125:1. My CFO wants some “facts” about staffing ratios before approving my headcount request. I would like to hear what your technician ratios are too. Thanks for any and all help!