I can't remember the specifics but I think someone mentioned that being one tooth off was worth X amount of degrees.

Also in your First post it was not possible for your your reading to have been "10 degrees before TDC".

When the Engine was built it was set at OT (top dead center) and as the Timing Chain Stretches and the Gears were the reading is after top dead center. If the Engine was running normally I don't see a way for the Chain to have gone before top dead center.

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84 300D, 82 Volvo 244Gl Diesel