(Probably easy) Help with javascript

I have a script that works great but I have a slight problem. I've added a new window.onload script outside of the one below, so now only the last one works. How can I run this code without using widow.onload?

<script type="text/javascript">
        window.onload = function calcPer() {
                document.getElementById("lbOverage").onchange = function calcPer() {
                document.getElementById("lblPercentOveralge").innerHTML = this.options[this.selectedIndex].text + '%';
                document.getElementById("lblOverageQty").innerHTML = Math.ceil((this.options[this.selectedIndex].text / 100) * document.getElementById("lblFinalOrderQuantity").innerHTML)
                var x = +document.getElementById("lblFinalOrderQuantity").innerHTML;
                var y = +document.getElementById("lblOverageQty").innerHTML;
                document.getElementById("lblActualProductionQuantity").innerHTML = x + y;
                document.getElementById("lblOverageQty").innerHTML = '(+' + document.getElementById("lblOverageQty").innerHTML + ')'
            }
        }
    </script>

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<asp:ListBox ID="lbOverage" runat="server" CssClass="alwaysontop" onChange="calcPer();" Height="85px"

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I have tried this:

<script type="text/javascript">
        function calcPer() {
                document.getElementById("lbOverage").onchange = function calcPer() {
                document.getElementById("lblPercentOveralge").innerHTML = this.options[this.selectedIndex].text + '%';
                document.getElementById("lblOverageQty").innerHTML = Math.ceil((this.options[this.selectedIndex].text / 100) * document.getElementById("lblFinalOrderQuantity").innerHTML)
                var x = +document.getElementById("lblFinalOrderQuantity").innerHTML;
                var y = +document.getElementById("lblOverageQty").innerHTML;
                document.getElementById("lblActualProductionQuantity").innerHTML = x + y;
                document.getElementById("lblOverageQty").innerHTML = '(+' + document.getElementById("lblOverageQty").innerHTML + ')'
            }
        }
    </script>

Select allOpen in new window

which works, however it does not work on the first selection. When a user makes a list box selection, it does nothing. Once they make a second selection, it runs as expected.