PX to % converter mixin. Example shows how you could build a grid with it. Can take any property so you don't need separate mixins for widths, margins or positioning properties (left, right, etc)

_pxtoper.scss

$page: 960; //This is our ideal width
$third-col: ( $page - ($gutter * 2) ) / 3;
$gutter: 20; //This is our gutter

@mixin pxtoper($property, $target, $context) {
    #{$property}: ($target / $context) * 100%;
}

project.css

.foo {
    width: 31.9444444%;
    margin-left: 2.0833333%;
}

.foo:first-child {
    margin-left: 0;
}

.foo-child {
    width: 48.9130541%;
    margin-left: 6.5217405%;
}

.foo-child:first-child {
    margin-left: 0;
}

project.scss

.foo  {  //Lets build a grid that has 20px gutters and three columns at 960px
    @include fluid(width, $third-col, $page);
    @include fluid(margin-left, $gutter, $page);
    
    &:first-child {
        margin-left: 0;
    }
}

.foo-child { //This is a child column to a 1/3 column
    @include fluid(width, 150, $third-col);
    @include fluid(margin-left, $gutter, $third-col);
    
    &:first-child {
        margin-left: 0;
    }
}