If you are setting up your radio for your first plane, it is recommended by many (including myself) to not use any expo, nor worry about dual rate settings. You want to get an idea of what 'raw' flying feels like. For example, picture yourself next summer when someone asks you to check out their new plane, and they haven't used any expo, and all you know is expo, you will find the plane incredibly 'twitchy' and not have the experience of flying a plane that way.

Now if you have been flying already, common settings for expo are around -20 (futaba) for low rates and around -50 (futaba) for high rates. Other radio manufacturers use positive to 'soften' the sticks about the neutral axis.

I use expo on all available channels (not offered for the throttle channel for obvious reasons). I set the plane up mechanically for max throw on the control surfaces, then tone down the end points using low rate function, the amount depending on your personal preference with that particular plane. You can start maybe at around 75% for low settings, and change according to how it feels to you. 75% should be enough to have enough throw to get out of a jam, yet be mellow enough for you to handle on a new plane with more throw than you're used to. Figure in the expo and you'll be fine. My rudder seems to have a lot of effect when I land, yet I like the plane on high rates, so I am at maybe 85% aileron and elevator, and 60% rudder on low rates, and 100% all on high rates. My expo is a little higher on my rudder on high rates, and I actually don't use low rates too often, maybe at the end of the day when I'm getting tired from the sun and trying to land low on fuel. If on high rates you still want more action out of the plane, you can increase high rate to 120 or so on the radio.

Hope this helps.