Never mind my previous post - I see that you know about the issue raised in it already. Guess I should have read all the replies first...

However,
although it won't help you analyse the current logs, if you could customize the logs to show the final byte offset of the range you could count a complete file download if the final byte offset matched the size of the actual file. There would probably be exceptions (I'm sure there are some perverted user agents that ask for the end of the file first) but it would be as accurate as any other method and a lot easier (I'm all about easier). Unfortunately, my lack of familiarity with Squid keeps me from knowing if it's possible to customize the logs in this way.