It's not that hard to calculate over a very long period of time. Simcraft has been doing it very, very well for quite a while now. Rawr uses a different model and a different system as well as typically much fewer iterations. Proc rate is known, what it procs off is known, the damage is known. Estimated up time over something like 10k iterations isn't that hard to calculate. All you have to do is figure actions per minute, along with the 2% proc and the damage output and duration of the Valks. You can have an as yet unknown numbers up at once (I've seen up to 6), so you don't need to figure overlaps, or other nonsense. You could even easily include an average life expectancy if you wanted to do that. There's a good reason you should take Rawr with a grain of salt (preferably attached to a margarita).