Interdependent dropdown med rigide værdier - JS

function changeOptions(strSource,intIndex) {
    if (strSource =="hus")
    {
        document.getElementById("beregningHus").selectedIndex = intIndex;
if(intIndex != 0) { intIndex += 2; }
        document.getElementById("beregningStang").selectedIndex = intIndex;
        document.getElementById("beregningFlag").selectedIndex = intIndex;
    } else
    {
        if(intIndex < 2)
        {
            document.getElementById("beregningHus").selectedIndex = 0;
        } else
        {
            document.getElementById("beregningHus").selectedIndex = intIndex - 2;
        }
        document.getElementById("beregningStang").selectedIndex = intIndex;
        document.getElementById("beregningFlag").selectedIndex = intIndex;
    }
}
function createFlagCalculator() {
    var aryHus = new Array("3","3-4","4","5","5-6","6","7","7-8","8","9","10","11","11-12","12","13","14","15");
    var aryStang = new Array("2","3","4","5","6","7","7,5","8","9","10","11","12","13","14","15","16","17","18","19");
    var aryFlag = new Array("50 x 38","75 x 57","100 x 76","125 x 94","150 x 114","175 x 133","200 x 150","225 x 170","250 x 189","275 x 208","300 x 227","325 x 246","350 x 265","375 x 284","400 x 303","425 x 320","450 x 340","475 x 358","500 x 378");
   
    var strHus = '<select id="beregningHus" onchange="changeOptions(\'hus\',this.selectedIndex);" onblur="changeOptions(\'hus\',this.selectedIndex);" style="margin:3px 0 6px;">';
    strHus += '<option>Vælg</option>';
    for (var i=0;i<aryHus.length;i++)
    {
        strHus += '<option>' + aryHus[i]
        strHus += ' m';
        strHus += '</option>';
    }
    strHus += '</select>';
   
    var strStang = '<select id="beregningStang" onchange="changeOptions(\'stang\',this.selectedIndex);" onblur="changeOptions(\'stang\',this.selectedIndex);" style="margin:3px 0 6px;">';
    strStang += '<option>Vælg</option>';
    for (var j=0;j<aryStang.length;j++)
    {
        strStang += '<option>' + aryStang[j] + ' m</option>';
    }
    strStang += '</select>';
   
    var strFlag = '<select id="beregningFlag" onchange="changeOptions(\'flag\',this.selectedIndex);" onblur="changeOptions(\'flag\',this.selectedIndex);" style="margin:3px 0 6px;">';
    strFlag += '<option>Vælg</option>';
    for (var k=0;k<aryFlag.length;k++)
    {
        strFlag += '<option>' + aryFlag[k] + ' cm</option>';
    }
    strFlag += '</select>';
   
    return "Husets højde til kip:<br />" + strHus + "<br />Flagstangens højde:<br />" + strStang + "<br />Flagets mål:<br />" + strFlag;
}
</script>

<script type="text/javascript">
    document.write(createFlagCalculator());
</script>