Complex projection in TCC

Summary

Trying to create a query using a filed on which a function has been applied.

Content

Trying to create a complex projection in TCC. Need to subtract a number from a field that has a function applied. That is --

First I am calculating the length of a field :

<quer:projection alias="testlength" xmlns:quer="http://www.taleo.com/ws/integration/query"&gt;
  <quer:lobLength>
    <quer:field path="Application,Candidate,EmployeeNumber"/>
  </quer:lobLength>
</quer:projection>

Next i want to subtract the number '6'  from the calculated length above. Unable to find a query on how this can be done.