For the accuracy check on the Digital Sharpening Guide — hold A on the SET TARGET screen. Print this page, cut out a wedge, and see whether the device agrees with it.
About printer scaling. Good news: an angle survives uniform scaling, so "Fit to page" does not change what these wedges measure — a 17° triangle is still 17° at any size. Print at 100% / "Actual size" anyway, so each wedge comes out at its designed size — a 160 mm base, long enough to rest the device on without overhang. The bar below should measure 100 mm at 100%; if it comes out shorter, your wedges are just smaller, not wrong.
Print at 100% (see the note above about scaling).
Cut out a wedge along its outline. Cut accurately — each wedge prints with a 160 mm base at 100% scale, and over that length 1.5 mm of wobble is about half a degree.
Glue or tape it to something rigid (cardboard, a cereal box) so it doesn't flex.
On the device: SET TARGET → hold A → lay it flat, press A, hold still.
Stand the device on the wedge's sloped face and read the number.
Within ±0.5° of the printed angle is a good result for a paper reference. If it's off by several degrees, re-do the flat capture on a surface you trust is actually flat — a kitchen counter is usually better than a whetstone.
15° — typical Japanese kitchen knife
17° — the firmware default
20° — typical Western kitchen knife
25° — Chinese cleaver / heavier edge
A paper wedge is good to roughly half a degree if you cut carefully. If you own a machinist's angle block or an adjustable bevel gauge, use that instead — it will beat this by a wide margin.