Thanks for the replies guys,

As far as the voltage expected from the torch, it will range from a maximum of 0 VDC to a minimum of -300 VDC... or visa-versa, depending on how you want to look at it. The relay is implemented to delay the activation of the scaler due to crazy stuff happening on ignition of the plasma. I'll also use it in the PIC code as an interlock for the scaler itself. I realized, after I posted this, that my son had a copy of ISIS from school. We ran a simulation on it as well, with the same results... -300 VDC in, 2.048 VDC out. I am no analog guy, can you explain why I need a split supply on the differential amp? Love to be educated on this.
In regards to the accuracy, the ADC will be 10 bit so... 300/1024=0.29296875 (1/4 volt or so).
Thanks a bunch guys,
Chris