For support I use 50 Staff to 1 IT. That will need to flex as a company grows so 50-60 is the best. Make sure your not geting too close to 70.
Larger firms tend to be more efficent and pamper less to users needs so figures of 100 to 1 and sometimes 150 to 1 can be common but they tend have long lead times for most tasks and dont provide a very responsive buisness.
400 Workstations and 1500 users will mean you will need to add a weighting to take in to account the extra training type questions you will get over an extended user base - Stuff like - How do I set up Out of Office etc.
Once you have worked that out you will then need to sort out your back-office staff.
Management of the Servers [Im assuming Windows Here] with little investment in tools to reduce the mundane tasks is about 25 Servers to 1 IT Member. With the right tools and experiance you can get about 50 to 1.
Management of the network is a tricky one - Do you have firewalls, VPN's, remote workers etc. Based on your supplied info the Network Administration could handle that workload but you will need to keep an eye on what volume he/she has so you can add extra staff if required.
Management of the department, board reports, liasing with Business heads, Business need research, Product Research, Support Management, Project and Change management, Documentation, Purchasing, Budget Management and general slack for Holiday cover means you will need at least 1 IT Manager.
These ratios will provide a balanced but effective department. Get that message accross and your manager etc can then say what response times, expectations should be delivered and you can then taylor it accordingly. If they want to reduce that service level [I.e server uptime, response times to requests etc] then you can show that by altering the Support Ratio etc. The last thing about this method however is that it makes justifying budget for department and IT staff investment really easy - something which can be difficult. For example - You want some software to monitor event logs and server health. This is currently done manaually once a week and consumes 2 hours. The software costs ?3,000 and if invested in would increase his ratio of capacity from '25 to 1' to '30 to 1'. The payback would be weeks and your department then has extra capacity [not doing a mundane task] to work on the nicer stuff. Works with training too.
Regards
John