Assuming that you also have a W210 with N22, you can read the high side from that. You also read the temperatures at the evaporator and condensor there.

You will want a low-side gauge that is calibrated in bar or psi, and it helps if it's accurate. You'll also need a hygrometer, an additional thermometer, and the charts in the diagnostic manual. There is not one correct set of pressure readings.

Note that even with the charts, you can only approximate the correct level of refridgerant in the system. But you can probably get it close enough to work as expected.

If your left side center vent is as cool as the right, I'd leave it alone. If it is warmer, add just a bit of HFC134a.

To add to the low-side port, you want to have the valve up on the container, so liquid does not enter the low-side line and slug the compressor.

Problem solved, not more hissing from the center vent area. Just added a can of 134a. Made no change to the gauge, showed 25 before and 25 after... so I cannot imagine that I would potentially overcharge it.
Works perfect... (for now )

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2008 S550
2010 GLK350
1997 E420 (retired)
2005 CLK500 (retired)