Have exising two wire thermonstat. Black and white wire. Wiring diagram different from acutual back plate (shows 8 possible wiring points.) Acutual back plate has only 5 plus double screw. Question is on the back plate, what screw do I use to hook up the black wire & what screw to hook up the white?

When using a two wire hook-up from the furnace the power supply ( usually red ) is connected to the RH terminal. Remove the jumper plate from the RH to RC unless you have two supply wires from the furnace. One being for heat (RC) and the other for A/C (RC). If that's the case then you have a three wire or more set-up. The feed back signal wire from the thermostat is connected to the W terminal and is usually white. So basically you have a power supply wire (red or black) and a return signal wire (white)