Best glide speed in the 737 is the driftdown speed which will vary with weight (anywhere from about 210 to 250kts near MTOW). Available from the FMC (CRZ page > ENG OUT > LEFT/RIGHT ENG) or the QRH Engine Inop Driftdown Speed Chart.
If I wanted a real time cockpit presentation showing my chance of reaching an aerodrome, I would put the aerodrome being considered into the fix page (if not aleady on the ND), then select the altitude on the MCP at which I would like to reach the aerodrome (worst would be the aerodrome elevation). The green "altitude range ring" (green arc) on the ND will then give you a real time indication (based on actual VS and GS) of the postion at which will reach the MCP altitude .If the arc remains between you and the aerodrome, find somewhere closer!!
You would be lucky to glide further than a 3 times profile in most instances so I would think that 3 times your current altitude (eg FL300 = 90nm max glide range) would be a good starting point to eliminate aerodrome that are likely to be too far away.
And I hope I never have to put the theory into practice!!