If you're still stuck, here is how you figure it out:
first, compare the calibration document to the will. Note how far apart the letters are first, and then how far apart the numbers are. In your example, where the calibration document is M20 and the Will is J19, the calibration document is 3 letters down from the Will (J to M), and the document number is 1 less.
Now, you have to work backwards to find a new calibration document that is the same number of letters & numbers apart from the Blueprints. You need to find a document that is 3 letters down from N, and one number higher than 17. Find that document, and save it.
Note: On subsequent playthroughs, the calibration document will always be M20, but the Will and Blueprints will have different letter & number combinations.
