Cut out the middle man - leave the expansion tank cap on, fill the system at the thermostat housing bung till its at the bottom of the threads or higher - any excess will either spill out as you tighten the bung or get forced into the header tank.... Then with the bung done up tight check the level in the expansion tank. Job done - works for me and I've emptied/re-filled my coolant so many times lately I've lost count......