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It is usually measured from the shoulder (ie base) of the reducer or flattener. Do not forget that every glass element between the sensor surface and the reducer will need to be compensated for in your spacer length. For example, if you have a 2mm filter, and the sensor has a 1mm cover glass, which is a total of 3mm of glass, you need to use the rule of 1/3s and add an extra 1mm to the spacer to compensate for the glass effect.
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