Robert's explanation of the film speed test is accurate and to the point.
By way of example, you might read a blank frame or the unexposed margin between frames to have an absolute density of 0.2, say. Then you would read the frames that are exposed in the way Robert describes to find one that reads 0.3, subtract the value you got for the blank frame (film base + fog), and you get 0.1--your Zone I density.
Sorry if "increased film speed" added another distraction. It is a common misconception that if you extend development time to "push" film to a higher EI, you are actually increasing the speed of the film. I had the misimpression that you were trying to adjust the Zone I density by changing the development time, which you can't do beyond a certain threshold.
By "pushing the shadows off the toe," yes, I mean moving them up to the straight line portion of the film's characteristic curve to improve tonal separation in the shadows. If the film's curve has a long enough straight-line area, you can do this without losing the highlights at the other end.
How much to change development time for a one zone increase or decrease?--depends. It's usually on the order of 20-40% The development time test based on Zone VIII will tell you. Once you've established the film speed (EI), then you can test to see which development times give you normal, +X and -X results.
What developer to use?--There is no particular reason to use the same brand of film and developer. Some, such as Kodak D-76 and Ilford ID-11 are virtually identical. Go to the Ilford website and download the technical data file for FP4+, and it will give you recommendations for different developers for different priorities (minimum grain, maximum speed, best acutance, etc.--you can't have them all!). Anchell and Troop's _Film Developing Cookbook_ also has lots of information on different types of developers.