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@gbj_tester
Vernier calliper is for a quick and dirty estimate; it assumes that the frame alignment is precise, i.e. that a plane orthogonal to the BB spindle and coplanar with the centreline of the seat tube will precisely bisect the rear wheel spindle axis. Since this is unlikely to be the case, it is necessary to set up the bicycle on a surface plate and make careful measurements with a dial gauge to check that the pitch circles of the chain ring and sprocket are precisely coplanar.