Paul, that stone looks like it won't flake well no matter how much heat you use. But it won't hurt the stone to heat it again. If it feels "powdery" or grainy on the surface after heat treating, it's a good indication that it's going to give you trouble.
Unfortunately, a lot of TX rock is poor quality. If you want the really good stuff, order from Bob at apocalithics on ebay, or from Kinley Coyan. There are others but those are the two guys I personally order from.