Hmmmmm..... I think the vehicle going straight would always have the right of way. I always yield to the car going straight, but I also go at the same time. As soon as they are clear of my intended turn lane, I complete the turn. Might not be the absolute "right" way, but that's what I do. I pretty much do the same thing at a 4 way stop.
I've been flipped off so many times by other drivers, I think I'm driving too defensively if I don't get the finger a couple times a week.
This thread is the very reason I love the roundabouts!
Just saying.....