alfa 147 jtd 1.9jtd 16v 140bhp lowering springs

Hi have got an alfa 147 1.9 jtd 16v with 17" wheels on it. Want to
lower it, any ideas which is the best make and how much should i lower it by. My car has had pi springs on before, the 40mm but my 17s rubbed so i changed it back to standard.
Thanks