Update:
I went to Prefs->Monitors and selected New, then changed 8 bit to 24 bit. I raised the pixelclock till the upper end so that I that I could achieve higer resolution modes.
On Horizontal timing, by reducing the sync and pulse length I managed to achieve 1440 (which was my goal here).
On the Vertical timing despite tweaking the values I could not achieve anything higher than 624 resolution (900 is my goal here).
I am clueless
This are my monitor resolution values taken from the Service Manual:
6. RGB Input (PC)-M198WA
No Resolution H-freq(kHz) V-freq.(Hz) Pixel clock(MHz)
1 720*400 31.468 70.08 28.321
2 640*480 31.469 59.94 25.175
3 640*480 37.5 75 31.5
4 800*600 37.879 60.317 40.0
5 800*600 46.875 75.0 49.5
6 1024*768 48.363 60.0 65.0
7 1024*768 60.123 75.029 78.75
8 1152*864 67.500 75.000 108.0
9 1280*1024 63.981 60.02 108.0
10 1280*1024 79.976 75.035 135.0
11 1440*900 55.5 59.90 88.750
12 1440*900 55.935 59.887 106.50
13 1440*900 70.635 74.984 136.75
That manual with further technical information is available at:
http://archive.espec.ws/files/LG%20FLATRON%20monitor%20ch.LP69G%20%20%20M198WA,M228WA%20.pdf