Why the formula use the size of the compressor ? Intuitively I would expect the size of the decompressor and compressed data is the only think that is really important to know how much the total information have been compressed.
Another way is to keep the file secret and just output a score, but if you want to use a public file, you have to include the program in the calculated size.
I don't think GP was asking about the decompressor, but specifically the compressor.
The FAQ has a fairly detailed answer for this though: http://prize.hutter1.net/hfaq.htm#addcomp