Adding data to new record with URL formula but having problems with order of operations

This is the formual to generate the next PO number:

var text jobNumber = ToText([Job Number]);
var number numberOfFollowingZeros = 4-Length(ToText([# of POs]+[Missing Sequence]+1));
var text followingZeros =
    Case
    (
    $numberOfFollowingZeros
    ,3,"000"
    ,2,"00"
    ,1,"0"
    ,0,""
    );
var text finalNumber = ToText([# of POs]+1+[Missing Sequence]);
$jobNumber & $followingZeros & $finalNumber



This is the Add button URL formula:

URLRoot() & "db/" & [_DBID_PURCHASE_ORDERS] & "?a=API_GenAddRecordForm&_fid_13=" & URLEncode ([Record ID#2])& "&_fid_21=" & URLEncode ([Next Purchase Order Number])  &"&z=" & Rurl()