Pop up window position at the centre of the screen

 Calling Pop up window and set the pop up window position at the center of the screen

function PopUp(targetPage, w, h) {
    var leftPos = 0;
    var topPos = 0;
    if (screen) {
        leftPos = (screen.width / 2) - 251;
        topPos = (screen.height / 2) - 162;
        var windowFeatures = 'height=' + h + ',width=' + w + ',left=' + leftPos + ',top =' + topPos + ',scrollbars=yes,resizable=yes,status=yes,toolbar=no,menubar=no,location=no';
        window.open(targetPage, null, windowFeatures);
    }
}

If you need to pass the querystring to pop up page

You can do in this way:

1. Set the Function on onclick

lnkUpdate.Attributes.Add("onclick", "javascript:SetPopUpParameters('" + lblTKNumber.Text + "')")

2. Set the querystring parameters and call the popup function

 function SetPopUpParameters(TxionID) 
        {
            var queryStringKeyArray = [];
            queryStringKeyArray[0] = 'TxionID';

            var queryStringValueArray = [];
            queryStringValueArray[0] = TxionID;

            PopUpWithQueryString('Page.aspx', 600, 150, queryStringKeyArray, queryStringValueArray);
        }

3. Call the pop up page by passing querystring

function PopUpWithQueryString(targetPage, w, h, queryStringKeyArray, queryStringValueArray) {
    var queryString = "";
    for (var i = 0; i < queryStringKeyArray.length; i++) {

        queryString = queryString.concat(queryStringKeyArray[i]);
        queryString = queryString.concat("=");
        queryString = queryString.concat(queryStringValueArray[i]);
        if (i != queryStringKeyArray.length - 1) {
            queryString = queryString.concat("&");
        }

    }

    var leftPos = 0;
    var topPos = 0;
    if (screen) {
        leftPos = (screen.width / 2) - 251;
        topPos = (screen.height / 2) - 162;
        var windowFeatures = 'height=' + h + ',width=' + w + ',left=' + leftPos + ',top =' + topPos + 

',scrollbars=yes,resizable=yes,status=yes,toolbar=no,menubar=no,location=no';
        window.open(targetPage + "?" + queryString, null, windowFeatures);
    }
}

Works in following