Here's a technique that works on Chevys, Fords...and Mercedes too!
Disconnect your vacuum advance. You want your timing to be about 36 degrees at full advance (the rpm at which it advances no more, probably 3000 rpm). Reconnect your vacuum advance and test drive. If there is pinging, retard the timing a couple of degrees. Repeat as necessary. Don't worry about initial timing.
This procedure assumes you don't have smog testing. (Factory settings are based on emissions requirements.) And it's only for non-computer controlled ignition systems.
Here's a link to a tech paper. Although it's written for Chevys, it works just as well on Mercedes (I wouldn't suggest something I haven't done myself...)
http://www.corvettefaq.com/c3/HowToSetTiming2.doc