I'm surprised that there are pull-down resistors on the input pins. This tutorial doesn't have that: http://make.larsi.org/electronics/TouchScreen/ I would expect the pull-down resistors would make everything non-linear and may be what is keeping your pressure equations from working.
This document talks about getting pressure readings:http://www.ti.com/lit/an/sbaa155a/sbaa155a.pdf
And I still do not figure out how to measure the resistance across the x plane (not the x position). This data is supposed to be provided by in the panel datasheet but I have only a range, not a specific figure. How should I do it ?