How could you ensure that all the spectroscopes used the same calibration line and equation? what would you need to be sure was always the same? what could be changed without affecting the calibration?

People had asked this many times and that is why they came up with methods and standards that will answer these type of questions. You can look it up in the NIST or the  National Institute for Standards and Technology.

Inot sure what it is i sorry : ((