Sim demonstrations - dual engine failure at 5000'/250kts/20 DME and 10,000'/250kts/35-40 DME.
Establish L/D speed and glide to the airport.
18:1 would give a range of 15 nm at 5000' and 30 nm at 10,000'.
Trading airspeed and flap extension at the latest possible moment allows you to reach the runway.
2nm/1000' would have you at 10 DME at 5000' or 20 DME at 10,000'. That's more than enough energy and no where near the max capability of the a/c.