I admire the initiative and thriftiness of doing these convesions, however, it is really important than when changing over, you evacuate the system as much and for as long as possible.
It is imperative that you remove all traces of R12 AND maybe even more important that you ensure that there is no moisture in the system. Moisture will combine with the refrigerant to create an acid that will eventually corrode the system from the inside out. This is particular important on most MB cars because of the huge amount of work involved in changing the evaporator which is the component most likely to be attacked by such corrosion. Changing the filter/drier during conversion is also a very worthwhile step.