After modifying an old coffee table I discovered the top was not flat but twisted. The method I chose to fix it was to fit a brace to 'push' the twist out. After determining the twist was across the diagonal and measuring the amount of twist, I fitted a brace with screws into the underneath with spacers at each end to push the two corners up. The thickness of the spacers is 1/2 the required push at each end. The centre screws must be fitted before the end ones, I used three screws in the centre spaced about 50mm apart to provide the required strength. The brace, in this case, could not go all the way across as the top mounting method prevented this.