Step 10:

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Keep your Photoshop document opened, grab your book, your ruler, a pencil… and a calculator !

On this pattern the 30 first sheets are not folded.

Zoom until the ruler show each pixel.
Place guides just before and after the first sheet (1st black column).

The vertical ruler displays 76 px and 81 px…. we have to divide them all by 12 !!
So, folds are at 6.35 cm and 6.75 cm (starting from the top of the sheet).