R406a is an approved R-12 replacement refrigerant composed by weight of 55% R-22/ 37% R-142b / 8% R-600a. It cools very well and has similar operating pressures to R-12.
The suction pressure is dependant on ambient temperature, relative humidity, engine rpm, etc. 40 psi suction at idle/80f ambient temp. sounds okay. I usually operate engine at 2,000 rpm for 60-90 seconds A/C blower on high and look for 22-30psi at 80f with your model car and R-406a. Under the same conditions, duct discharge temp should be less than 45f.
Hope this helps.
1980 240d 4sp
ASE Master Auto Technician
ASE Advanced Engine Performance