I don’t think you have to tune memory for just 200 users. I have 200 users ant it runs fine in a virtual machine with only 512 MB RAM, though it is running on a CLI only linux box.
I have never tried to tune Openfire’s memory so i’m not sure, maybe it doesn’t have to change on that page. Anyway, you shouldn’t look at this page as it doesn’t always show the correct info for the current moment. Even if it shows 99% this is only true for a short time and this is normal. You should only worry if your server begins to crash and write a lot of out of memory errors in the logs.