The rough rule of thumb is to add two teeth in the rear for the equivalent 29er gear. But the gear inches will help you nail it exactly. For the 26" I run 34x17 (2:1) and on the 29" I use 36x20. Both work out to be 52 gear inches.
I have a 33 tooth chainring, 18 cog and 29 inch wheel - not counting the final diameter with the tire - that means my GI = ~53.2. Compared to where I've been - and my destination - I'm currently not in very good shape. Based on many of the trails I've ridden thus far, this bike seems geared "low", but the GI seems a bit higher than the "standard" 52. The bike shop sold me 180mm cranks, but I'm still unsure how the crank length would fit into the equation . . .
. . . this is too much analysis for me . . . I think I'll just go riding and enjoy what's hooked up and rollin'.