Windows 8.1 multi-mon DPI scaling
Folks, if you want to learn how Windows 8.1 does DPI scaling across multiple monitors with different DPI, see this blog post:
Windows 8.1 DPI Scaling Enhancements
Windows 8.1 has an algorithm that takes the manufacturer supplied info about all the panels attached to the system including resolution and physical size, and adjusts the scaling factor on each monitor so that they match as closely as possible. There are some limitations with this approach and the behavior is quite different than Windows 8. For performance and other reasons, the individual monitor scaling factors that are automatically chosen are those familiar steps: 100%, 125% 150% 200%, etc. You can't select arbitrary scaling factors on each screen individual to get things to match 100%.
When you drag a windows from one screen to another screen with vastly different DPI, you'll see an effect where the window scaling dynamically shifts as you move your mouse over the threshold. Its a little weird at first, but I find it works a lot better than the old Windows 7/Windows 8 way.
I like to use my Surface Pro right next to my main monitors because I don't like wasting that beautiful tablet screen. My big screens are older Dell 1920x1200 24" models, so they have less than half the DPI of the Surface Pro: