'Turning downwind' does not establish a position. A 'Downwind' call when abeam the upwind end of the active runway tells everyone else where you are and gives a clue as to the timing of the rest of the circuit (ok, unless you are on a B52 circuit).
Hmm, I dont know?
I would hope pilots would fly the crosswind leg accurately over the numbers so turning down wind should provide an acurate enough estimate.
I have always liked the terms early downwind and late downwind with everything in between simply being downwind (early downwind being up to the upwind threshold and late downwind being beyond the arrival threshold).
What do others think / do?