A reasonable present rule of thumb for heavies is that they glide (nil engines) somewheres around 2nm/1000ft .. which approximates 12:1.
Slight thread drift, but doesn't 'glide' mean no thrust, ie all engines out?
I agree with John_tullamarine I have seen 2nm/1000 feet work to perfection several times (in the (real) simulator
) with both engines out on various series of 737.