Output first page of doc as 20mm high jpg

Hi there,

I need to output the first page of several thousand documents as 20mm high jpegs (at 300dpi).

Chinnadk kindly helped me get this far:

var doc = app.activeDocument;       w = doc.documentPreferences.pageWidth; 
doc.viewPreferences.horizontalMeasurementUnits = MeasurementUnits.pixels; 
doc.viewPreferences.verticalMeasurementUnits = MeasurementUnits.pixels;  
app.jpegExportPreferences.jpegExportRange = ExportRangeOrAllPages.exportRange;  
app.jpegExportPreferences.exportResolution = 300/w*72;  
app.jpegExportPreferences.pageString = doc.pages[0].name;   
doc.exportFile(ExportFormat.JPG, new File(doc.fullName.toString().replace(/\.indd$/i,".jpg")), false);

but that was before I realised I needed it 20mm high.

Any help much appreciated.