Imagine a grid, or a spreadsheet: letters along one axis and numbers along the other. You have free access to get the will but not the blueprints. You can also see that to reset the machine, it needs to be at M20. That's the origin point, and it'll travel a relative distance from that point to get the will. In your case, from M20 to O31. That's two letters over and 11 numbers down. You want the blueprints and you can recalibrate the origin point, so change it so that getting the "will" from this new origin point will instead retrieve the blueprints. Two letters back and 11 numbers up from blueprints' P29 will be N18, as you've found out.
Or, if you want to think of it another way, take the difference of the co-ordinates between the will and the blueprints and apply that difference to the origin point M20.