The reason it is so hard to estimate is that as soon as it isn't secret anymore they are expelled and thus no longer *in* the congregations. I would say that according to the definition of the CCoJW, more than half of the Witnesses who regularly report field activity are apostate.
I have never met one that did not have his or her own beliefs about one or another doctrinal point, just waiting for the "chariot" to catch up to them. They dare not speak it openly, but they all have them as far as I can tell. Any who diverge from those codified doctrines in their hearts are apotates, having drawn away from the teachings of the Faithful and Discreet Slave.