Pace wrote:
Hence I find it hard to see how a flight level can be used with accuracy without reference to the actual pressure or at worst regional pressure setting
Pace, I know you know this
, but FL measures atmospheric pressure (albeit in units of feet), so it is exactly what the regs should specify.
It is the altitude which is less accurate, though as Backpacker said, altitude may be more practical if the TA is high.
Pilotage: agreed, the body can even over-rule the equations!