Uptrim Disable,
MMEMatty is correct when he says that min drag speed would give you best glide ratio. As to where to find it, it would be very close to best holding speed or best angle of climb speed, both of which are available from the FMC. Best hold speed is also available in the perf manual. A deviation of + or - 20kts or so would probably have little effect and I personally would shoot for about 240kts in a 737, which may be on the high side but would give better windmilling and a little more energy for turning, etc.
On a 747, I would go for 240 - 280kts, depending on weight.