My 18" AMG alloys are on a 2001 W210 specified with differently sized tyres to the dimensions you give - 8x18 rims 245/40 front tyres and 9x18 rims 265/35 rear.
Up to 130mph: front 32psi, rear 33psi on light load; full load: front 35psi, rear 42psi.
Above 130: front 36psi, rear 36psi on light load; full load: front 41psi and rear 48 psi.
All the above recommendations are taken from the filler flap and are for cold tyres. Add up to 4psi for warm tyres according to the flap.