Expo has nothing to do with how fast your servos are. It's for when you need the occasional big throw at the extreme ends but find the center to be too sensitive to be able to fly comfortably and smoothly. I find that about 20% is a good place to start. It softens up the middle just a bit without making the ends feel sensitive.
As for your Cub throws, no way should you need 28 degrees of throw on any surface. I assume that you mean degrees instead of percent anyway. Cubs need a bit more rudder than your trainer does, but even then it's easy to get into a bind by overcontrolling the plane with too much throw. For the first flight, I'll recommend splitting the difference between low and high rates and adding 25% exponential in the right direction for your radio. Use that setting for your initial trim flight only just so you know you'll have enough throw to bring it in if your trim settings don't go far enough, then back it down to low rates and keep it there. Since you're very new to piloting in general and to Cubs in particular (which have been known to make guys with 2-3 years experience commit mild acts of blasphemy more frequently than any of us would like to admit) I'll offer a few tips.
1. Wait until you have a windless day. Cubs suck in the wind.
2. Use a couple of yardsticks or something similar to make sure the wheels on the main gear are straight. Some guys like to run a 1/2 degree of toe in, but I find that straight wheels are a bit more forgiving. Even with that, Cub main gears are not at all a forgiving design so you need to get it as straight as you can.
3. Make sure that after aligning the main gear properly your tail wheel tracks straight with the rudder also straight. If you don't, you'll fishtail that thing all the way to the scene of the crash.
4. Choose a windless day. Cubs suck in the wind.
5. Try to fly when there isn't much wind, as Cubs are very sensitive to the wind.
6. Delay flying the Cub at all until you're comfortable using the rudder. It's optional on a trainer, but mandatory on a Cub since the tail is so much smaller and the fuselage is so short compare to the wing.
7. Have a long runway. Cubs are floaters on the landing approach, and you'll likely overshoot your first few dozen landings.
8. Control your lift on landing with the throttle. It's the right way to land always, but a Cub with that giant wing will let you pull the nose up and stall it right into the ground whereas a trainer will just drop its nose.
9. Don't depend on the plane to self-correct. A Cub, if anything, will self-uncorrect. You have to be on the sticks and keep it flying in the direction you want it to.
10. Don't try to make it into an aerobat or a hot rod. Cubs are wonderfully relaxing planes once you get to know them, so let it be that for you. I can't stand to see guys stuff big power plants into them and then pull huge verticals and consecutive rolls with them. There are planes for that, but let the Cub be that slow lumbering floater that it was designed to be.
11. Don't fly it on windy days until you are very comfortable.
12. Expect a pronounced left yaw just as you advance the throttle and before the tail comes up and compensate with right rudder. Release the right rudder as the tail comes up and let the plane track straight on its own. Then give it a touch of right rudder as you rotate to take off. If this takeoff procedure doesn't give you a predictable and straight takeoff, then either you skipped step 2 or 3 or your runway is impossibly rough.