AoS Teleport Astrolith Bearer

The most common houserule is to measure from base to base. This creates problems, because in this way your clanrats can attack my Gyrocopter, which doesn't make much sense.

But "official" rulings (to measure distances within the models) don't help.

By FAQ, flying models count as being as high up as their flight stand allows. If an enemy model can't reach the model from the ground, they are immune to their attacks.
To attack a flying model that is out of reach, you need to climb a nearby scenery.
The interesting thing is that if you cannot climb a scenery with your move, you must remember how far it has climbed each turn: basically, FAQ allow you to proxy where your base is. (FAQ 1.2, page 3)
FAQ say also that vertical movement is free for flying models, "the vertical distance is not included when working out how far they have moved".

The most logical consequence is that a flying unit can proxy its base to be in the air as high as you want. You can move your flying model by it's move and say that it's at 20" high from the ground (since vertical move is free).

But basically no one plays it that way, as there's no real ruling about it, just hints in the FAQ. At most they measure the distance between models, and flight stands are not that high... so we'll remain with the nonsense of clanrats that attack gyrocopters.

 

Airborne units that are flying high, cannot control objectives.

I measure base to base, but it's equally frustrating for an army as Kharadron overlords to have undermined their combat style because everyone can melee their airships...