As a rule of thumb, we've long assumed you have a glide range of around 100 miles after an all-engine power loss at cruise, although obviously a heavy aircraft at 30k isn't going as far as a light aircraft at 43k. If you're over land, that will generally give you some options of where to put it down.
I'm a bit surprised that an all-engine power loss isn't normally trained for since it's something we design for (Ram Air Turbines, 30 minute batteries, etc.) at the aircraft level - even if it's classroom stuff such as the best glide speed.
Prior to Sully's little adventure in the Hudson river, I remember a debate over the value of rafts and life vests on jet aircraft since there had never been a successful ditching of a big jet (although to be fair, the only attempt I'm aware of was the Ethiopian 767 hijack, and that's not a very good data point since the hijackers were apparently fighting the flight crew during the ditching attempt).