Hello Brad,
Using either Z1 or Z2 should be OK if you only need to tell if there is no-touch/noise/very-weak-touch or a true/strong touch. Note that the Z1 value from the TSC should be 0 if no touch or very small if noise/very-weak-touch; and the Z2 is the max (=4095 if 12-bit resolution) if no touch or very large if very- weak-touch. You may read from the TSC the Z1 or/and Z2. and then test with your touch panel and system to find a proper threshold.
Regards,
Wendy F.