Don't worry too much about the accuracy. I used both CorelDraw and Adobe Illustrator to draw a grid over your original JPG (starting from the top left and ending at the bottom right), and doing so revealed that the JPG's graph varies from the computer-generated one by almost 1% at the far end:
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(Green lines are the AI grid.)
I confirmed the results by using Meazure and found the CDR and AI divisions were accurate to within 0.1mm, while divisions in the JPG varied by 1mm.