514.3? I got 413.8 hmmm, math must be off. But to be honest, and not to be mean, there is no way at all to give you an average, rough guess, or anything else. What you need to do is sit down, and list out all of your plans (mainly space and bandwidth but you can do other little tricks for the use of php, mysql, etc..)
Now lets assume you have 1000GB of transfer a month (just a guess at a common number)
If you were to offer plans that allowed 500mb storage, 5GB of transfer, its nothing more than simple division to give you how many plans you can hold on that machine without overselling (you did just want a rough guess, not an exact calculation right?)
Put as many as you can, but once you hit 200, watch the server loads at for a few days. If it still looks fine, increase in 50 ish customer increments, still watching server load. I'd suggest that once you hit 75% of server specs + bandwidth sold, i'd get a new server.