The p-node retuns the co-ordinates of the point in universe space in 1/10th inches units: x-co-ordinate in red channel, y-co-ordinate in green channel, etc. so to have say output 1 at 1 foot use multiplier 1/120 = 0.00825

note Poser units are 8.6 foot. 

input values for the transparancy channel are scalar.

color inputs are converted to scalar by averaging the rgb channel values, so your output will be -1*(0+0+-2)/3 = 0.666667, your transparncy therefore should be 0.5*0.6667 = 0.333333

Inputs outside 0-1 are clamped to the nearest limit.