Although the 998R is not mentioned in this article by Brad Black on setting up the
non-linear TPS, I'd be pretty sure that it will be the same as the ST4s and S4R which use the same TPS and 5.9 ECU. The TPS is still set at 150mV with the throttle fully closed and the idle stop backed off but you will have to "backprobe" the pins at the TPS to get a connection between the black/violet and orange wires.