Brave measures the percentage of your Reaction abilities to work. For more information, search for the Battle Bechanics Guide in GameFAQs. If I'm not wrong though, they gimped the guide and made it simpler to read, but less info than before...

So if your reaction ability is Blade Grasp, chances if it succeeding is equivilent to your Brave. If your Brave is 97, chance of Blade Grasp succeeding is 97%. Same for Hamedo and the Split Damage thingy that calcs have. There may be modifiers for certain reaction abilities, but there is none I know at the moment.

The Formula for Faith is written in the Battle Mechanics Guide. Just look under the Spells section, they should have the formula there. You basically take the base formula for that particular spell cast on your and modify the result with the formula for Faith given in the guide.

The highest you can go with Brave and Faith is 97 and lowest is 3. This is because every 4 Brave/Faith points you increase during a battle via Mediator or Beowulf, 1 point will be permenant. Do the maths from there . Unless you use the FFTastics or FFTedit tools or some other editor.

And...I don't think there's an advantage for low Brave.