You really need to know your shutter speed variations. If you have a roll film back, one test roll is usually sufficient to establish the variations.
If you don't have a roll film back, a "
PhotoPlug" type of device and a smartphone will be helpful - (these are only useful for central shutters, curtain shutters requires a different way of measurement). If you are comfortable with a soldering iron, Google will direct you to instructions on how to make it from recycled parts and pennies.
Buying or making - either way will cost you less than one or two sheets of Portra film and processing.
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