You are totally misunderstanding me. The units ID uses internally are postscript points which are 72 per inch. It makes no difference what ruler units you use, at 100% ID is going to base the view on showing you the equivalent of 72 postscript points in the size it "believes" represents one inch, which is calculated by checking the number of pixels being displayed and dividing by the reported system resolution (I'm making a presumption, here).
Two different monitors may both be running at 1920 x 1200, for example but if one is a 24 " and the other is 27" the apparent size of what is on screen is going to be very different, even though Windows or OS X is using the same value for resolution in the OS. ID can't tell the physical dimensions of your monitor, so it displays the rulers using the same calculations on both monitors, and the size will be wrong, but differently wrong, on each one, unless the the "true" resolution of the monitor -- the number of pixels displayed divided by the physical size of the display -- is exactly 72.
In order to find the correct zoom level for a lifesize view you need to calculate that "true" monitor resolution and divide it by 72, the number of postscript points you want to display in one real inch of monitor space. It has nothing whatever to do with 72 ppi or 96 ppi beyond what those numbers may be doing to affect the size of the pixels on the screen, and you don't need to know what number the OS is using.
The second post was probably my misunderstanding you. Again, I don't believe the system resolution is particularly relevant. It's a constant, or at least can be treated as a constant, and your display depends on the physical size of the monitor and the number of pixels it displays. I keep hearing that Windows uses 96 ppi, and I don't understand where that number is even useful, if it has a real meaning, or is simply a relative value and not truly tied to a physical dimension (which is what I suspect). The number of pixels that will be displayed in an inch in your browser will still be determined by the same monitor resolution value/physical size calculation, modified by any zoom factor.
The same 600px wide image displayed in the same browser using the same settings will use the same proportion of the screen regardless of the physical dimensions of the screen if two monitors are set to the same resolution. If you change the monitor resolution setting, the image will be rendered larger or smaller and will take up a greater or lesser part of the total browser window.
