I'm trying to determine how many pixels down I've scrolled using
The cross-browser compatible version for
document.documentElement.scrollTop. Please see the 'Notes' section of this piece of Mozilla documentation for a full, detailed workaround in IE8 and before.
As mentioned here,
pageYOffset is another alternative to window.scrollY (note though that this is only IE9+ compatible).
In regard to the link above, check Example 4 for a fully compatible way to get the scroll position (it even accounts for zoom as @adeneo mentioned!) using