Ah - there's a riser height calculator on the AVS forum:
Riser Height Calculator - AVS Forum | Home Theater Discussions And Reviews
The idea of that is to give the people in the back row, an uninterrupted view of the bottom of the screen, but as I said, having it higher is probably not as good for surround sound. With yours, if you have similar height people front and back, can those in the back can see the heads of those in front just breaking the bottom of the screen? Or is your screen high enough to avoid that?
I'm thinking of going a little below the numbers you get from the calculator, with the idea that when we have both rows in use, the taller people are at the back.