This is a strange little topic. I frequently see it discussed and it exemplifies the risks associated with 'a little knowledge'! As per previous posts, if you have x:y, then you have total of (x+y) parts. The number of people who swear that there are 'y' parts makes a person worry about intelligence levels. After all, we use the term '50:50' in every day language - it doesn't work by the alternative argument.
At the end of it, dilutions are almost never more than a guide. People like me simply pull them out of our hats, test if they are ok, and go with it. If it is a bit stronger or weaker, it is not going to do any harm to anyone!