I can't answer your question specifically, but if it's a mechanical injection pump the timing is usually a static setting for installation on the engine. Timing adjustments for RPM are typically done by a centrifugal weight mechanism on the drive gear or by a centrifugal advance mechanism internal to the injection pump.

The manual timing of 15˚BTDC is likely the static timing for installation on the engine and is likely all you should really be worrying about.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
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1986 500SEL Euro (Rusted to nothing at 47K!)