Custom fluid on non-fluid stuff. Example will loop from max-width: 1003px to max-width: 800px calculating the current number for the width on each step.

fluid.scss

@mixin customFluid($start, $end, $startValue, $endValue) {
    $diff: $end - $start;
    $valueDiff: $endValue - $startValue;
    @for $i from 1 through $diff {
        $currentStep: $diff - $i;
        $currentValue: $endValue - ( $valueDiff / $diff ) * $currentStep;
        @media (max-width: 800px + $currentStep) {
            #start-page {
                width: $currentValue;
            }
        }
    }
}

@include customFluid(800, 1004, 80%, 100%);