I don't think your software 'knows' how large 100% will be on the screen. Just imagine that you change your screen resolution setting from whatever it's now to 640 by 480 pixels... the displayed photo would become larger, just as the buttons and other GUI-elements on your screen.
My best guess is that 100% means that 1 pixel in your bitmap will be mapped to 1 pixel on your screen.
If the displaying application is a bit more sophisticated, it may also take into account the dpi (dots per inch) that's stored in the image metadata. You can measure the dpi of your monitor and store that in the image metadata and see if this changes the way the image is displayed.
Otherwise, you'll have to do some more involved calculations to get it right.
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