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
After you get X and Y readings you'll need to take two samples to calculate pressure:
Set +Y HIGH and -X LOW. Do analogRead() at +X for Z1 and -Y for Z2. (measure repeatedly and average to reduce noise)
Note: if Z1 = 0 then Pressure = 0
Rz = Rx * X/1024 * (Z2/Z1-1); // Note: Rx is the resistance across the X plane
Pressure = Poffset - Pscale * Rz; // Poffset and Pscale are determined experimentally.