Bro before getting it done, I guess its better to lift the car from the rear via Jack by the same height you want to add to rear and then check the effect on front height... If the car is at your desirable level, then get it done... Measure before and after lift height readings properly.
I think front height reduction effect is most probably in the case where the rear height is exactly the same or more than the front height... Only then front height will reduce when you lift more from the rear.. If rear height is less than front, then increasing rear height will have no effect on front height...!
