If supply pressure (to meet regs) is 21+/-2mbar at meter outlet, and maximum allowed pipe loss is 1mbar, I make that minimum pressure at meter outlet 19mbar, and 18mbar at the boiler.Not necessarily
But I just googled it make sure the 21+/-2mbar is at meter outlet (as I'd expect), and and there is a wide range of opinions, some saying the pressure required in practice at most boilers is much lower than 18mbar. Ask 10 GSR guys and you get 11 opinions!