Had the warm driver's side/cold passenger side syndrome. Added about three quarters of a can of R-134a today. Didn't hook up the gauges to the system yet, but I will. Helped to bring the driver's side up to close to what the passenger side is. They both read about 54 degrees with the climate control unit set to 70 degrees. How low of a center vent temperature should I expect to see when everything is up to spec? Also, at what point should I be thinking about either adding some oil or completely draining the system and refilling with a proper freon and oil charge should it leak down again?

Len
'99 E300TD 93,000 miles

When it's up to spec the center vent should be the coldest vent at around 39.00 - 42.00 degrees F while at 1500 rpms.
As for your last question, that compressor is not cheap. You should make sure you're not leaking oil and fix ASAP.