Capt M for an aircraft gliding at 200+ knts from 30,000+' the gliding range may be significantly altered by the prevailing windspeed at the different levels. For yer average Cessna / Piper / Robin etc at best glidespeed of 65-ish knts and 1,500-2,500' height, the best range is academic. In the latter circumstances it's best to pick a field that you can reach even if the windspeed at low level is 30knts against you. If you end up too high then you have the option of picking another field or adjusting your height to suit your first choice. Best range would only really be applicable in an SEP (or light twin) if you are within a reasonable distance of an airfield and you need to know if you have the height to make it.