I stole this from a Stevebfl (moderator) post :

I have looked at hundreds of MB timing marks (at the cam) and they are only
accurate enough to know which tooth to line up. There is 18degrees between
each tooth. If you marked and followed your own specific motor you might could
form a judgement. To just drop in and view it isn't good science.

The proper way to do it is very simple. The valve clearance is removed and the
engine rotated till the intake valve on number one cylinder is pushed exactly
2mm down by the cam lobe. Any resonable dial indicator will measure this.

When the motor is in this position the crank position is read from the crank scale
and indicator and compared to the book value. In the case af a 617.95 motor
with cam ID 11 it should say 11deg ATDC. If it says 21deg then there would be
ten degrees of stretch and a new chain would be in order.

I threw that 10deg increment out because I consider that much to be obvious,
it might take 350,000 miles to stretch one that much though and if asked I say
replace it at 200k. I very seldom measure chains by anything more than mileage
and noise. I would replace anything in doubt over 100k. (ALL mileage numbers
are my opinion only!!) (The timing numbers for cam ID 11 came from 1985 TDM)