ONDRA_PATA: If you want to go really advanced, here's the whirlwind breakpoints :
http://i.imgur.com/3BmaOqd.png
What this means is , to give a random example. Having 2.25 Attacks per second is exactly the same for whirlwind as having 2.40 attacks per second, as they're in the same breakpoint range. At those points you can either drop some attack speed without losing aything, or get some more to hit the next breakpoint.
The reason why the breakpoint thing is hard to balance for this build is because of pain enhancer, what some people like to do is to balance the breakpoints around havingg one stack of pain enhancer, so they're balanced for the rift guardian.