I want to caution you about using a half or one wavelength line to do
measurements. That's a viable method if the imedance of the antenna is
close to the characteristic of the line. If it's not, you'll find that
even a surprisingly small line loss -- one that you'd normally consider
negligible, can seriously skew your results. The calculation is
straightforward presuming you know the loss -- I'm sure Reg's program
would be adequate. Do some what-ifs with various antenna impedances and
you'll see what I mean. The effect gets worse as the antenna and
transmission line impedances get more different, and as the line gets
longer. That is, a one wavelength line will have more effect than a half
wavelength one.

Also, line length becomes more and more critical as the impedance of the
antenna and transmission line become more different and as the line gets
longer. Again, a little experimentation with the calculations will
illustrate what to expect.

Even if you carefully account for the transformation of the connecting
line or don't use any line at all, you have to be aware of common mode
currents and how your test setup differs from your normal rig
connection. And finally, even with a perfect lab setup, you'll find that
good impedance measurements can be hard to make with amateur equipment.

Before you get carried away, make some measurements on the bench with
your meter and using good quality loads, or at least RC combinations
using chip resistors and capacitors or ones with extremely short leads.
Make impedances similar to ones you hope to measure. If you can get
values which are accurate enough to suit you, go to the next step and
measure the same loads through a transmission line as has been
suggested, and see if you're able to extract the actual load value from
the measured value with sufficient accuracy.

If you get that far, you've partially answered your question about what
kind of accuracy to expect, and you're ready to start figuring out how
to deal with common mode currents.

Decent antenna impedance measurements aren't simple to make, even at HF.
They're more difficult at VHF and above.

Roy Lewallen, W7EL