JPEG Compression parameters in Descartes 2004

We need to merge a lot of raster maps (in pairs) using Descartes 2004,  producing JPG+JGW output files.
We have implemented a batch MDL/C++ procdeure sending Descartes Commands in synchronized mode, for instance:
IMGTRANSFORM OUTPUTFILE SETNAME f:\home_prj\mergeRaster\prova5.jpg
IMGTRANSFORM DIALOGRUN SETOFF
IMGTRANSFORM MERGE SETMETHOD MERGE
IMGTRANSFORM MERGE SETSOURCEPOLY 0 4000000
IMGTRANSFORM MERGE RUN
We also need to control the Compression Factor of the resulting Jpeg file through the Command
IMGTRANSFORM OUTPUTFILE SETCOMPRESSION ???
In the Interactive Dialog Box  the Compression Option Button  suggests
JPEG Minimum/Low/Medium/High Loss
and we have to know the corresponding parameter to pass to SETCOMPRESSION command

Thanks, Alessandra