The formula of deecibel calibration

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I did some experiments about Audio Amplitude , but  I have three question about calibration.

Questions

• What is "Calibration offset"? 
  
• What is the formula of "Calibration offset"?
  
• What is the formula of Refebrence SPL , and how is it calibrated from the original value?
  

If you know something about these question , send posts here , please.