You should allow the Brett to work in the secondary for at least a month if its already in the beer. let the Brett eat all the residual sugars in the beer before you bottle, and then add sugar as usual. If you are just going to add Brett at bottling time...you should have a pretty dry beer first, and then bottle in something with thick glass...like the Belgian 750s. If you have a high FG, the Brett will eat a lot of that sugar and turn your normal bottles into bombs. So, my suggestion is that if you have a high FG, put your beer in a secondary fermenter and add the Brett and leave it there for a couple of months to let it dry your beer out before you bottle.