Internet Explorer 6,7,8 problem with font-size in percents : font too big

Hey there,
Not sure if my title describes well the problem (see attaches file for seeing my problem : left is IE8 rendering right is FF3.5 rendering of the same page on a Windows 7 system).

The problem is that by default (from typography.css) Internet Explorer is rendering font very huge and Firefox just as good.

To tweak that problem i've used ie css files included inside fusion-core/css (ie6-fixes.css, ie7-fixes.css, ie8-fixes.css) and add these lines (i've choosen a 14 pixels size by default on the theme page so...) :

/* hack for big font in ie problem */
.font-size-14 {
  font-size: 12px;
/* font-size: 87.5%; */
}

note for readers : if you've used a font-size 12 by default it would be .font-size-12 to tweak for example
I've putted 12px since it's the value that make the display approximately correct for both browsers.

My question (since this size is a parameter that could be changed inside the theme page of Drupal administration) : Is there a better way to achieve the compatibility ? Or is there a way to handle percents font inside Internet Explorer better ?

The original problem seems to be that we don't give a based pixel size for rendering so if ie have by default a 16px font-size and firefox another (say 11px) : the problem of size will be always there and the rendering will be always random.

Not sure what is the best way to handle this.

Any advice will be wellcome for handling this correctly
Regards,
Denis