I personally don't believe that the line up markings are distinct enough to judge chain stretch. The proper way to do it is with a dial indicator on the proper valve. At 2mm of lift there are specifications for crank position. The crank markings are distinct and thus are the measure.

The upper rails do not need the cover to be removed but everything mounted to the front must be removed to expose the rail pins.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician