Trying to add 1080p 48Hz.

[ToastyX]

You're overcomplicating this. The manual only lists the rates for each resolution, not what it actually supports. Forget the manual. Just add 1920x1080 @ 72 Hz with "Exact reduced" timing. If the monitor won't accept that, try increasing the horizontal blanking to 120. That's still under 165 MHz pixel clock. As I said, the monitor might not accept 48 Hz, so it probably won't accept 40 Hz, but you can try anyway. The lowest is probably 50 Hz.