Stephan,

Try using the postitive stop method when trying to determine exact BDC. I made a stop using an old glow plug and a long bolt, rounded at the end. I place the degree wheel on the crank, and eyeball it to BDC. Then insert the stop so the piston hits it around 30 ABDC. Write this down, Then go back to BBDC and read the degrees from pointer, Write this down. (this figure might be 35deg or 25deg, doesn't matter) Now add the two figures together and divide by two. This will be the point where your pointer should be for absolute BDC.

Example:

Set pointer at 0 deg BDC and tighten degree wheel to crank.
Adjust pointer close to 0
Install positive stop and set it to 30 DEG BBDC or TDC for that matter.
Rotate engine one way and measure deg. =31deg
Rotate engine Other way and measure deg. =27deg
31+27/2=29 Original setting was 30deg. 30-29=1deg.
Now adjust your pointer 1deg by bending your pointer, and remeasure

When you get 39ABDC and 39 BBDC or 30 ABDC and 30BBDC or whatever then you know you are dead nuts at 0BDC.

I hope this helps, and I hope I havent confused anyone!