This is a nice little challenge you're facing with this, i like that! :) Right now I'm developing something similar, sort of an rotational wheel like control with inertia spin and "rubber-banding" when reaching a certain threshold etc.!
If i got you right, you're using a UIScrollView for your control AND you're trying to use/abuse the already existing bouncing "feature" of the UIScrollView right? Do you have a problem with calculating the speed values based on the scrolling speed or does the kicking-in rubber-banding causing problems?
Did you consider implementing the rubber-banding effect on your own, via "touchesBegan.."/"touchesMoved"? Mimicking the rubber-banding is quite simple: from a certain threshold value on the on-screen movement of the view would be half the finger-drag movment until total stop. see also: Mimicking the "rubber band" effect from iOS in the Gallery on Android
You could also check out the implementation of iCarousel maybe. It has similar "rubber-banding" effects custom programmed.
I know this is not an satisfying response/answer, but maybe it points your thoughts into some other direction.