Do you mean an 'idle power descent' or a 'no-engine glide'? Many of the glide angle figures being quoted are more appropriate at flight idle and the 'real gliding' L/D will be substantially less, I would hazard in the 15-18:1 range.
Normal holding speed is a good point to start but in reality, the speed for best glide (under power) will reduce as the residual thrust increases.