H202 offers good advice here...

I have the RIV and lock-pin tools, In my experience the lock-pin tool will get you close enough for the engine to run OK but it can still be 3-4 degrees off the published 15 ATDC spec. I have not tried the "center the raised mark in the hole" approach to see how accurately that works. My guess is that it would not be as accurate as the lock-pin approach.

Several very knowledgable people here have said the bubble method will work on the OM603 engine, even though that method is not described as a valid timing procedure in the OM603 book. Hopefully they will see this post and offer a bit of guidance.