How to use calculated values (xmin,xmax,ymin,ymax) as input for a a wfs/wms reader bounding box or search envelope.

Since there is a limit on download I need to add a search envelope on my WFS and WMS import. I got this working in 2 ways: using the search envelope in the parameters of the reader or the XML filter expression.

"

<Filter>

  <BBOX>

   <ValueReference>Geometry</ValueReference>

   <Envelope srsName="urn:ogc:def:crs:EPSG::31370">

     <lowerCorner>159221.09606242698 220934.16757763177</lowerCorner>

     <upperCorner>160221.09606242698 221934.16757763177</upperCorner>

   </Envelope>

  </BBOX>

</Filter>

"

My question is how I can use calculated coordinates as an input for this reader. Since the coordinates of the bounding box will change.

(I know it has something to do with user-parameters but I can't get any number in the expression or parameter of the search envelope.)

Thanks