BLP shared JPEG header

Recently I got the problem to write JPEG compressed MIP maps of BLP properly into file, so I've searched around and found this thread which doesn't really help me since I still don't know how I have to write down the JPEG header. There's always one shared but I think there must be some other header data per MIP map where I get the size from since when I read MIP maps from Warcraft III textures I only use the jpeglib to read their size which is always correct although I've added the shared header before the MIP map data itself.
Could someone please explain me how I do have to store the MIP maps properly? I am writing OGRE and Qt plugins for BLP and I do support one thread per MIP map for JPEG compression already.