There is no way you will be able to tell how much memory it will use.
Exchange is effectively a big database, so it will try and get as much in memory as possible. That's not to say however that if your mailbox database is only 600MB it will only use 600MB of memory. Exchange has a ton of "other" services it runs apart from the information store, all of which can use varying and considerable amounts of memory.
Put Exchange aside for a moment, and Windows needs memory to run stuff, your antivirus requires memory, your backup agent will require memory. What I'm trying to say is numerous other things running on the server all require memory, and if you and I buy the exact same server, we'll install different bits and pieces on it and it will have a different memory footprint.
You can run all the calculators you want and read all the guidelines, but they're just that - guidelines. Personally, these days I won't get a server with less than 12GB of memory in it, because in the overall scheme of things, it is relatively cheap and if I were you, I'd try and get some more in your box.