I had been struggling with bed flatness on 310mm round Rostock MV2 bed. I have ended up removing the PCB Onyx heater, the melamine support, and mounting a flat 1/4 inch Al plate directly (with spacers and heat shield under the heater) to the base at 3 points. My previous (stock) setup was warped, and it was making the boro glass to warp too. However, it seems that the boro glass has become flat again after sitting free for a couple of days. Currently, I am using PEI over the boro glass on the top of 1/4inch Mic6 Al. Although my bed may not be absolutely flat, it is a great improvement over what was going on before. After 13-point calibration with the mini IR, the bed leveling reports error of around 0.005mm vs over 0.1mm before. Applying the adhesive to the glass was certainly tricky, and I had to start over several times. Applying 0.8mm PEI was easy.
In my experience a 3 to 6 mm thick glass will conform, at least to a degree, to a surface under it, so if the bed has a dome in the middle, the glass will warp in the center