The answer is -- no simple answer. I've been doing performance tuning & capacity planning for the last 5 years of my 25 year IT career. Here are some things to help you:
1) shared ethernet should be loaded to 30% of capacity (i.e 10Mb can carry3Mb without collisions starting to intrude on service)
2) switched ethernet will work at rated capacity.
3) identify a profi;le for your "typical" user -- or even better, more than one. The point is to identify the workflow(s). One might be "power user" with lots of large file attachments and over 200 msgs a day. Another might by "light user" with only 10 msgs per day.
4) categorize the users at the locations by profile.
5)Estimate the traffic (i.e. bytes) associated with the load (number of users of each type multiplied by the load for that type)
6) add 20% for overhead
7) Divide by the number of hours in the "average work day". This is the bandwidth you would need for 1 average hour. Divide it by 3600 and that's your bandwidth per second for an average day.
8) Determine how much spare you want (i.e 50% extra for heavy times), and multiply by that factor. You now have your Peak Bandwidth needs in bytes per second including allowances for LAN/WAN engineering loads.
9) allow additional capacity -- calculated similarly - for Internet surfing, ftp/downloads, etc. It's a safe bet that there's probably more than just NOTE/email going over the WAN.
In defining your user profiles, some other factors to consider are replications, private databases and broadcast/grouop messages.
While a "magic number" is what you want, it isn't that simple.
"Give a man food and he eats once -- teach him to grow food and he'll never go hungry." - some smart guy a long time ago
Hope this helps.
