Good old trick :-)! I went one step further by taking measurements in-situ before removing an old mainspring.  There is always at least one good spots to stick in the caliper's jaws while mainspring is still inside the barrel :-). Then divide by 10, 11, etc and voila! Measuring just one "coil" with  such caliper as shown below (in mm) is not always appropriate. How do you measure 0.125mm mainspring when your caliper has some limitations with resolution and jumps from 0.12mm to 0.13mm?
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