Michał and Gluglug, thanks a lot! I understand much better now! I did not understand why the value was hardcoded while people were mentioning it should be calculated. Sorry it was a stupid question, I should have checked the old doc :-(
I will flash with a non free VBIOS and report it to the list so that it may benefits others.
BTW, If anyone wants to swap the X60 tablet screen for a 1400x1050 IPS screen , I can tell the name of the ebay seller who is accepting "buy it now" at $85 (while others only sell the screen for over $250), but off list as I would not want to cost him too much money and be told that I "advertise" for someone :-). He is a very courteous seller, selling brand new screens and not used ones.
This is could be interesting to others because the only thing that can age in a X60 is the screen, turning yellowish. The 1400x1050 uses a CCFL backlight, but Xiph (from VLC) has instructions on how to replace it with a LED backlight on
https://people.xiph.org/~xiphmont/thinkpad/led-inverter.shtml (haven't done that yet)