Saddle shape mesh (V-Core 3.1 500mm)

I started with a big dip at "D" after following the advice from the video I ended up with a saddle shape. It looks like the bed is bowing down at the unsupported corners.
The total range is 0.390mm when heated to 70C
I verified that the Y rails are parallel to eachother and they were screwed in the correct order.
Any suggestions on how could I improve this?
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