Um, are you talking about an extract top off batch or a full volume ag, pm or extract batch? The only time a grav reading is "unreliable" is if it's a top off batch. If you're doing a full volume boil, the the gravity is going to be the same all the way through the wort, since nothing is being mixed up.
If it is a top off extract batch, and you topped off to the correct final volume, as I say in
this thread, you just go by what your recipe says it should be. It's foolproof. If it's AG or Pm, and you are topping off, if you took a pre- top off gravity reading, you can use a dillution calculator like the one in beersmith to estimate it. Actually even if you are doing an extract top off batch, if you were to dump the wort into your fermenter and it had graduated volume marks, you could take a gravity reading of the undiluted wort, the input that, with your current volume, along with the intended volume, and selecting "dilluting with water" and the dillution calculator would tell you what your gravity will be once it's topped off.
SO really, if you think about it there's really no point where a hydromter reading has to be unreliable.......And the other time, if you've topped off with water and didn't do what I just said you just need to go with the hydrometer reading of the original brewers of the recipe....so even then there's not instance where a hydrometer is unreliable.
You just kinda need to know HOW it is reliable in any given situation.