On my E-class car, I could see 5 small dots on the original spring pad (shim) when I shone the flashlight at the top of the spring. 4-dot pad is 4mm thinner than 5-dot, 3-dot pad is 4mm thinner than 2-dot, ... (1 inch = 25.4mm)
The pads are not that expensive, last time I asked, it was something like $10-$20 each pad at the dealer. The expensive part is the tool, a special spring compressor ($200) to compress the spring so you can remove the old pad and put in the new pad. Or have some auto shop do the work.
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