I assume that the area the ball pitches and the point of impact are fairly accurate as they they actual events, but the predictive element of where the ball would have gone had it not hit the pad has a margin of error.
Personally I would get rid of the umpire call element of the system when overturning decisions. I don't see that the on field decision should be determinative if you are backing the technology
i would retain umpires call for things that are predictive, like hitting the stumps. but make add in a margin of say 10% of the ball must be hitting for it to out - even if already given. i dont like the 'once its given all it has to do is graze the stumps' as the technology isnt 100%
i think im correct that in football the middle of the ball has to cross the line, why cant they just use that for hitting the pads, and just remove umpires call again.
there was an interesting piece on sky at one of the county games with Nas and Wardy where they showed that umpires call effectively added an extra stump on either side.
I would like to see the use of DRS as simple as possible, to avoid all confusion