UIRefreshControl: how to track critical deviation

I would like to change the appearance UIRefreshControl

after the user has pulled it enough to trigger the refresh action. For example, I would like to have a "pull down to refresh" header assigned that goes into "release to refresh" when the dropout reaches a breaking point.

Currently the only solution I can think of is overriding continueTrackingWithTouch:withEvent:

from UIControl

and tracking vertical movement from there. That being said, I would have to start some guesswork and testing to find the correct distance (which can also be device dependent). Any idea?