Guessing development time or using times other then from your own personal testing is like shoppiing at The Good Will, you just never know what you'll find or how it will fit you. The best way to determine development times is first by understanding the questions and THEN to pursue the answers.
Film speed is directly related to development and developer, which is in turn related your choice of paper/developer, which in turn is related to the effect you want your prints to have. So start at the very end - the final print. Do you want to make "ordinary photographs" - that is, photographs that convey a literal representation of the subject matter photographed? Do you want a grainy, rugged look & feel? Or would you prefer fine-grain, sutble tones and smooth transitions?
Figure out what you want your final print to look like, and you're ready to proceed to the next step - deciding on a film/developer combination. Once you've researched the various films & developers around and decided on a combo that seems promising, you start "testing".
In testing your film it is important to understand what it is you're testing for. Exposing film to light produces a latent image on the film which is subsequently brought out in development.
The amount of exposure you give your film primarily affects how the shadows in your scene - that is, the important darker areas will be rendered. If, after printing your negatives you find that your shadows are just too dark and hold too little detail, you need to rate your film at a lower ISO setting so as to provide the shadows with additional exposure. If the opposite is true, then a higher ISO setting may be in order.
Development (developer, agitation method, concentration of solution, temperature, etc) primarily affects how the highlights in your scene - that is, the important bright areas will be rendered. The point to altering development times or methods is most often intended to affect a change in the highlights. Increased development, agitation, temperature or concentration of the solution will yield lighter highlights on the print, and vice versa.
The combination of exposure and development effectively stretches the contrast of the scene. The important thing is to MATCH that tonal stretch to the paper/developer combination that you are using. Though most papers today perform exceptionally well, they aren't identical in this regard. Some paper/developer combinations will hold a greater "dynamic range" - that is, the range of tones (from jet black, to paper white) that the paper is capable of containing.
The objective of testing film is to obtain a negative with range of densities, from jet-black to paper-white, that when exposed to paper for a given time, will print and look as you would like them to.
I know I haven't really answered your original question in this thread, but in reading it, I couldn't help but wish that, years ago, someone would've given me the response that I've offered you here.
Good luck!
