I've found Brett very unpredictable. I've run the same strain of Brett-B with similar worts and had attenuation's between 67% and 80%. It is very frustrating.
For the one that only went 67% (1.060 to 1.020), I tossed in half a cake of PacMan yeast after a couple of months to finish it off. I don't have my notes, but I think it bought it down to a respectable finish around 1.010.
I was concerned that it might cause the Brett to act as a secondary yeast, but it didn't. It had none of the 'rustic' brett flavors, even after a year in the bottle.