Depends on what you mean by error. Is it a linear system? If the near
field strength increases with no change in the radiating structure
itself (and propagation is stable, etc.), does the far field not
increase by the same ratio? But of course, with a repositionable
(rotatable) directional antenna, it's pretty hard to calibrate the FSM
in a meaningful way since the antenna system changes (quite a lot, with
respect to the FSM) as you rotate it, so you don't know from one time
to the next that you have the RIGHT field strength. I'd (ideally) like
to know that the transmitter is properly adjusted to output a clean
signal, and that the antenna system presents the proper load to the
transmitter, AND that the antenna system is radiating like I'd like it
to. The "SWR meter" is one component that helps me, but with only one
of those tasks. (And yes, it's fine with me if you care also about the
SWR on your 450 ohm balanced line...there may also be good reason for
wanting to know that.)

Cheers,
Tom

PS--Frank, if you look back in the archives from this group, you'll
find directional couplers (of the sort that measure the line at a
single point) explained in great detail with four-part harmony and the
whole nine yards. Go study them and you may understand why calibration
is important.