Advice On MIS Dept. Staffing

I’m an IT Manager for a private, non-profit company. My CEO is asking for some sort of IT standard or best practices recommendation on adequate MIS staffing based on network user size.
I am unaware of any such thing, but asked a few of my peers (two of which are consultants). They recommended at least 2 support staff. Our current user size is 110 users (and growing). We are on a W2K Server network, and are in the process of implementing a new accounting software system. I am involved heavily in this implementation, and am trying to prove to her in writing that I need more staff. I use a local company to help supply hardware and software, but a good deal of the troubleshooting is left to me.
Help!