yeah the hidden screw is for cars with a seperate heatersystem, dieselcars most off the time(night heater that runs on fuel)we(m5) dont have that i think.but not sure.
normally you just turn on the heater on slowest function. open the radiator and fill it with liquid up to 5 mm above max coolant on the radiator, then close it and run the car to optimal temperature. then let it cool down and look if the water is lowered,iff so ,refill again to 5mm above max and do it all over again. thats what stands in bmw TIS.
hope this helps a bit,