The print dialog matters
A document that looks correct on a laptop can emerge from a printer at a different size. Page-fit options, margins and printer defaults can change the scale. Our ruler is intended to print at 100% or Actual size, with automatic fitting turned off.
Check a known length
Before using the printout, compare its entire zero-to-100 scale with a physical millimeter ruler. The distance must be 100 mm. Do this on every new printout, including one printed on the same machine with a different paper setting. A visual guess is not a scale check.
Keep the sheet usable
Use a clean, flat printout with readable marks. Wrinkled, stretched or poorly printed paper makes small markings harder to interpret. Do not place sharp paper edges near the eye surface. If the printout is not comfortable or readable, use a proper physical ruler or request professional measurement.
Treat printing as preparation
Correct print scale does not establish correct technique or a clinically verified PD. The print button gives you a measurement aid. It does not remove the need to understand the intended use, preserve the original labels and seek help when the result is uncertain.
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