Or, if you don't want the hassles of overlapping Big Guy bases, you could make the squares 40mm X 40mm. I've made 5 boards now doing that, and it has worked out quite nicely. The main thing you have to do is make up your own Throwing Template; I built 2 from acetate sheets and PK5 Drawing Board. If you measure all the lengths of the smaller Throwing Template (Quick Pass, Short Pass, etc.) and multiply them by 1.33 (you've added an extra third to the squares), you'll come out with a Throwing Template for 40mm squares that is still exact. Comparing the smaller and larger Throwing Templates, they cover the same amount of squares.
However, a 40mm square board is a tad bigger...
-Vilegrimm