Hello,
For what it's worth, from the Ponton factory manual(translated from German): 15 to 25mm of freeplay measured at steering wheel rim is acceptable. The later cars with manual or power assisted steering only list the 25mm(1 inch) maximum. I would therefore take 15mm as the minimum(?)
The actual adjustment is a FRICTION adjusment with the box removed from the chassis and bolted to a fixture. I agree with the tech as many drivers that have been driving front wheel drive cars with rack and pinion steering will find a Mercedes steering vague and the car difficult to control in a fast corner.
Before attacking the box to make adjustments or to replace it, check the steering column bearings and the steering coupling, a hard rubber disc in a W123 and strange metal/plastic bush assembly in W107/108/109/110/111/112/113/114/115/116, these things do wear out after 20-30years.
Have a good week.

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Nachi11744