scaledToWidth

The metric set scaled so that one row of the Start screen — margin, wide tile, margin — is exactly width across.

This is the scale a surface should normally use, and it is the reason a Metro layout has no breakpoints: Windows Phone stretched one 480-pixel canvas to WVGA, WXGA and 720p rather than reflowing anything. Passing a factor to scaled by hand means choosing a number instead, and a number chosen by eye cannot be re-derived — the desktop demo ran at a hand-picked 1.6 for a week, and the fitted factor for its 560 dp window turns out to be 1.64.

The divisor is read out of the metric set rather than written down, so it stays true if the tile sizes ever move. It is 342 dp and not the 360 dp the canvas is wide, and that difference is unexplained: 480 Metro pixels is exactly 24 + 432 + 24, which says the Start screen's outer margin is 24 px where margin is the 12 px of a text page. Until somebody reads that off a real Start screen, fitting the row is the honest thing — it makes both edges equal without asserting a number nobody has verified.