Hi Brian,
I think you missed the point of my message.
Microsoft offer no "official" suggested support levels.
The industry seems to average around the 1000 users per administrator but your business has a budget and a level of importance it places on Exchange.
These two, along with Office Politics and the Administrator's competence, will determine your business' staffing levels for Exchange.
Just because *survey* XYZ says "Industry averages suggest you need 1 Exchange Administrator per 1000 users" wont mean YOUR business does. Stated another way, there is no "true standard" applicable to any/every situation.
I've done a fair bit of surfing and have only found anecdotal evidence of staffing levels for Exchange. As I opened with - the only important recommendation would come from Microsoft - and they dont.
Hope this helps,
I expect its not what you want to hear, though.
Cheers,
Tim.