Windows Graphics Programming Win32 GDI and DirectDraw第六章疑问

《Windows Graphics Programming Win32 GDI and DirectDraw》6.1节中有这样的描述：

The Windows NT/2000 graphics engine uses signed fixed-point numbers to represent coordinates, having a 4-bit fraction part and a 28-bit signed integer part. Points with negative coordinates or coordinates beyond the height or width of the device surface are considered clipped. So the maximum physical device size is 2²⁷ by 2²⁷, or 0.88 mile by 0.88 mile in 2400 dots-per-inch (dpi) resolution.

我自己算的方法是：2^27 / 2400 / 1000 = 559240.55 

作者给出的是0.88mile，感觉很疑惑。

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