JavaScript - Calculate two textbox values on TextChange Event ClientSide

js function:

<script type="text/javascript">
    function findaverageL() {
        var l1 = document.getElementById('<%= txtqnty.ClientID %>');
        var l2 = document.getElementById('<%= txtNCRqty.ClientID %>');
        var avgl = document.getElementById('<%= txtOKqnty.ClientID %>');
        avgl.value = parseFloat(l1.value) - parseFloat(l2.value);

    }  
   
// this function is for validate Number Only. It not allow to entr characters in textbox
        function isNumberKey(evt) {
            var charCode = (evt.which) ? evt.which : event.keyCode
            if (charCode != 46 && charCode > 31
              && (charCode < 48 || charCode > 57))
                return false;
            return true;
        }
   
</script>

<asp:TextBox ID="txtqnty" runat="server" Text="0" onkeypress="return isNumberKey(event)"       onpaste="return false" ondrop="return false" onchange='javascript:findaverageL();'></asp:TextBox>