Lots of good stuff in the posts so far. It depends what you are trying to do.
You may be trying to glide the best distance (to reach an airport a long way away), or you may be trying to stay in the air as long as possible (to diagnose the problem, start the apu etc).
Best glide range will be at S/E driftdown speed which is in the manual, and also in the FMC, on the classic anyway. Just remember that as this will probably be below minimum clean, you will have to ignore normal flap speed schedule!
Minimum sink/best endurance speed will be about 0.2xV.S. below this,(ballpark 20 kts) but this is not in the books.
From trying it in the sim, I reckon a reasonable rule of thumb is: twice the height for estimating range, instead of normal three times. So from Fl200 you will get about 40Nm in a straight line.
Mike Sykes gives some excellent guidance on the Chris Brady website:
Loss of thrust on both engines.