Thanks, Trentino and for reminding me of the principles of flight and mechanics. The combination might involve some mind blowing mathematics... I have Handling the Big Jets and very good IMO.
I agree that perhaps my question wasn't as succinct as it might have been.
What's been keeping me awake at nights is, assuming no wind (hypothetical I know), no air traffic restrictions etc. and lets say a jet close to maximum load, then surely there must be a distance at which it's not worth going up to 35,000 ft because of what it cost to get up there both in fuel and time. I just wondered what that distance was, approximately?...