Aliases in calculations for MySQL
It's more readable if you use a derived table. No need for 4 SELECT clauses, just 2. Plus, it's proper SQL, not some fancy feature that MySQL or Access has added:

SELECT InvoiceNumber,
       CalcField1,
       CalcField2,
       (CalcField1 - CalcField2) / CalcField2 AS Percentage 
FROM 
    ( SELECT InvoiceNumber,
             Field1 + Field2 AS CalcField1,
             Field3 + Field4 AS CalcField2
      FROM ...
    ) AS x ;


The other option, is not to use a derived table at all but double the calculated expressions:

SELECT InvoiceNumber,
       Field1 + Field2 AS CalcField1,
       Field3 + Field4 AS CalcField2,
       ((Field1 + Field2) - (Field3 + Field4)) / (Field3 + Field4) 
         AS Percentage 
FROM ... ;