This will vary based on personal tastes, flying style(s) and plane.

First, you should set your control surfaces such that you have the total throw you need. No more, no less. This is an important step in setting up D/R and expo.

Think about what you want in stick control. Most precision flying is done around center, be it take off, landing, level flight, traditional aerobatics, etc. For these maneuvers, you want precise control over the control surface, i.e nice soft sticks where large stick movements give small and precise surface response. Aggressive flight (Snaps, spins, tumbles, 3D) requires larger throws, but by the very nature of more aggressive maneuvers, the precision of the throw isn't as critical as other types of flight.

When a typical radial arm servo is coupled with a control surface in the typical way, the setup is most aggressive around neutral. This makes the control surface sensitive around center position and more precise at full throw. This is opposite of what is typically desired. For this reason, expo is typically beneficial for any setup.

As for expo amount, I believe a figure around 15% will give linear response. Since there is a benefit to softening the center beyond linear in most cases, a good starting expo is at least 30%. I typically crank expo up so that the stick feel for the less aggressive flying feels nice and smooth, and then when I want full throw, it comes in quickly. If you put in too little expo, the sticks will feel "twitchy" around center. If you put in too much expo, the sticks will feel "out of control" or "too aggressive" at 3/4 stick or higher. The key is to find the expo sweet spot. This will probably vary by surface and may even vary by deflection direction.

As for D/R, I used to use them all the time. Now I basically never use them. With D/R, you can easily get into trouble if you try a maneuver in the wrong rate. The only time I can see D/R being useful is if you cannot find a good expo setting due to too much throw needed, such as possibly 3D flight. Even if you do use D/R, you should still dial in expo for both high and low rates to soften the center.

As for quantity... I found that I like very similar expo between planes, regardless of intended flying. I.E., I use basically the same expo on my 2M pattern ship as I do on 3D planes. At first I found this strange, but it does makes sense as my personal taste for control feel would be the same across multiple planes, i.e. I need the same expo across multiple planes. I do however use a 3D rate on 3D planes.

Expo I use on all planes is around the following. Aileron - 50%, Elevator - 50%, Rudder - 60%. I found that for me, this gives a very nice feel. Out to about 3/4 stick, plane response is very precise and smooth. When I need full or very high throw, it shows up quickly past 3/4 stick, but no so quickly as to be a suprise.

Another related issue is throttle curve. You will have the same servo issue with throttle, but this is compounded by engine response not being linear. On most engines, with the carb 1/2 open, the engine is already making 3/4 of its total power. A throttle cure will allow you to smooth out stick feel on throttle.

EDIT - I was side tracked writing this and I didn't see RCKens post until after my inital post. I'd agree that since trainers fly like wet sponges, expo is probably a waste. However, for most second planes, expo may be helpful. I really don't see the point in learning to fly with sticks that are agressive at center and soft at high throw when there is a simple fix. It would be a lot easier to learn if setup is more precise and easier to control. For this reason I don't see expo as a gadget, but rather an important flying tool. I have purest flying buddies that would disagree (some don't even trim thier plane), which is fine by me, but I'd go nuts without expo and a few simple mixes. However, expo can bite you if you set it up wrong, either too much throw to begin with, too much expo, or expo in the wrong direction. For this reason I would suggest no more than 30% to start and getting some help from a more experienced pilot at your local club. You don't just set expo and call it good. It may take many flights to dial in a comfortable feel. I typically need 20+ flights before I am happy with plane/radio configuration.


Cheers!