I usually assume brett will work SG down to 1.002 in the bottle, so figure in the difference between your current gravity and 1.002 when calculating priming sugar weight. If you want to be conservative, just assume 1.000 as FG.

Since berliner weisse is normally consumed rather quickly (like any light wheat-based beer), I would only be concerned about additional carbonation if you're going to cellar a few bottles for 6 months or more. In that case, just fill a few heavy duty bottles.

Just so I'm clear, if I plan to keep the bottle for less than 6 months then calculate the amount of priming sugar in the usual way. If more than 6 months, factor in the difference between the current gravity and an assumed gravity, say 1.002, in the priming calculations. Did I get it right?

Do you have a recommendation for a priming calculator capable of factoring in the gravity?