Pre-and Post- thermostat. Pre-Thermostat would be considered pressure (between the waterpump and thermostat). It is a bit of fuzzy math, particularly when the thermostat is closed and flow is through the by-pass hose.
That coolant drain plug would flow coolant, but is pretty small. What size line do you need? Just as important, where will you return the line? Your best bet maybe to tee into the heater hoses. The lower one off the pump is the higher pressure.
