ie6 + position:fixed nugget

Anyone who hates css expressions .. feel free to leave now, and end up doing the same thing in a more convoluted manner in 'regular' javascript. If it eases your conscience any -- I know it does mine -- this css is sourced in a conditional comment just for ie6

#my-fixed-position-element{
    top:expression(Math.max(document.documentElement.scrollTop + (document.getElementsByTagName('html')[0].offsetHeight / 2), 20) + 'px');
}