Several people who have used a dial indicator have noticed that the bed appears to be bowed in the middle. We can set the corners to be the same height, but in between the extremes of the X-axis the bed seems to come up about 7 thousandths of an inch. This apparent bowing is believed to be due to sagging of the extruder support. To test the assumption and to compensate, I installed counter weights that offset the weight of the extruder assembly. With the counter weights, the apparent bed bowing came down to 3 thousandths. With the counter weights in place, I was able to print a large, thin flat surface that had not turned out well without the weights.
The counter weights are very low-tech, but they work. I used pulleys and fishing weights suspended from an enclusure that surrounds the TAZ. To balance the lateral forces in the X-direction, I split the weights and mounted pulleys on the etreme left and right. The extruder assembly is suspended with about 11 ounces of weights on each side. I used fishing line to suspend the weights, so it is hard to see in the attached pictures.
Hope you find this helpful.