That is why they use entomologist's in some cases. They can pinpoint the time of death closer to the actual time.
Medical examiners can give a pretty accurate time of death if they get to the body soon after death. But if it has been quite awhile before they discover the body they tend to give a wide range of time for death.
I wouldn't say pinpoint. It's been pretty well established that there are a lot of variables that go into the calculations and they are estimating, too.