I found that you have to vary for each browser:
This works great in safari and chrome:
zoom = parseInt(document.defaultView.getComputedStyle(document.documentElement, null).width,10)/document.documentElement.clientWidth
This works well in IE8+ (unless your user has some freakish screen....):
zoom = window.screen.deviceXDPI/96
And for Firefox and IE7 I use this
although I still haven't got it to work with Firefox on the Mac...
Good luck.
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