As another data point, which conforms to
kenparry's estimate, on a BA A319 flight CDG-LHR (~185nm) we flew at FL240, according to the captain (I was pax that trip). Of course, once we got over London, we then had to fly holding patterns for 20 minutes.
From another angle, commercial jet climbs and descents to/from FL300 require
approximately 100nm each (3.3 miles per 1000 feet) - but that is definitely a
rough rule-of-thumb that will vary with aircraft, weight, ATC, winds, most efficient cruise speed, etc. etc.
But those are practicalities separate from absolute best fuel economy calculations.