Keep in mind that Barbacide is the FINAL step in cleaning. First, you want to remove the surface gunk and soap scum with Simple Green, Scrubbing Bubbles, or other means that are described in other threads.
I just checked my bottle of Hydrocide, and they recommend 4 ounces of solution per gallon of water. This works out to a ratio of 1:32.
A capful per 8oz of water should be about right, but don't worry if you use slightly too much or too little, as the ratio is not critical.
Let the razor sit sit for 10~15 minutes, agitating occasionally, then rinse it thoroughly.