problem may occurs coz validators maintains anonymous function for onblur event in which its not able to get control which ur sending in parameters..
try by making function without params...
i.e.
function SetStandards() {
var Conducted = document.getElementById("<%= txtTecQua0.ClientID %>");
var Standard = document.getElementById("<%= txtTecQua1.ClientID %>");
var TargetControl = document.getElementById("<%= txtTecQua2.ClientID %>");
TargetControl.value = Number((Number(Standard.value) / Number(Conducted.value) * 100 * 10) / 10).toFixed(1);
if (isNaN(TargetControl.value) == true) {
TargetControl.value = '';
}
if (TargetControl.value > 100) {
alert('Some error message.');
Standard.focus();
};
};
and call it on blur like
onblur="SetStandards();"
hope it will help you. :-)