Double-spaced lined paper
Half an inch between lines, roughly double standard notebook ruling. The space is not for your handwriting — it is for whatever gets written between the lines afterwards.
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Paper that expects to be marked up
Corrections, a teacher's comments, an editor's pencil, a translation written above a source text: anything that will be annotated should be written on paper that has planned for it. Standard ruling forces the annotation out into the margin, where it loses its alignment with the words it refers to and becomes a puzzle to reconnect a week later.
Interlinear work
Glossing a foreign-language text — writing a translation directly under each word — needs a full clear line per line of writing, and so does music theory homework with harmonic analysis under the staves. So does any first draft you intend to work over in pencil. In all three cases the second line is doing real work, and it has to be directly beneath the first, not beside it.
A motor-skills accommodation as well
Wide-spaced ruling is a common and effective accommodation for dysgraphia, and for anyone whose letters wander vertically. It removes the need to control ascender and descender length precisely, because there is nothing immediately below to collide with. Pair it with a light line colour so the ruling guides the writing rather than boxing it in.
Printing it accurately
This only measures correctly if your printer doesn't rescale the page. In the print dialog set Scale: 100% — not "Fit to page" or "Shrink oversized pages". If you're not sure it worked, print the calibration page, measure the ruler on it with a real ruler, and it will tell you exactly what your printer is doing to the page.