I would actually set Xms == Xmx, as you can easily fill up your entire heap... why let the JVM manage total heap size in a tool when you can predict the heap size? (For the record I use ~2g Xms and Xmx for Java projects, and ~3g Xms and Xmx for Scala projects).

Also you can ditch MaxPermSize if using Java8, which you should.