I had the same question when I was doing my layout. I looked at different "commercial" 7-button layouts after I couldn't find a real diagram for any layouts of this kind (6-button layouts/dimensions are everywhere, but 7th was harder to find for me), and the differences in location for the 7th button had a lot of variation from one product to the other. At least from what I could tell from the pictures...
The above advice is a good idea. I just played around with the spacing and kept printing out layouts to see what felt comfortable for my hands to go along with the standard 6-button spacing. I ended up going just a bit closer than the optimum distance for my thumb, since I figured not everyone would have the same size hands as myself. I'd rather have it a little too close than too far away.