The steering box is one of the few achilles heels of the 123 car.
According to one of the experienced MB Techs who frequent this site, MB advises against adjusting the steering box. Even if you are successful in adjusting it, you will not be able to get ALL the play out.
The only alternative I know of is a new or rebuilt box which is quite expensive.
I expect, from your description, that you have already checked the steering linkage to ensure that it is tight. Since it is only loose in the center, I expect that it is tight.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in