Your coax is what, 10-12 feet long? That's pushing a couple of
wavelengths on 2m, but isn't quite there.

To read the impedance of the antenna directly and get a good set of
resistance and reactance curves clearly, you need probably less than a
foot of coax, or an exact multiple of an electrical half wavelength (so
exactly 2 wavelengths * velocity factor would do it). You can use your
MFJ-259 to cut the line.

It's possible to transform your impedance readings into the antenna's
impedance if you know the (random) electrical length and loss of the
coax and are handy with a Smith chart or have some other calculating
ability.

Exact multiple of a half wave is easier... it's an impedance repeater,
aside from the loss in the line. To that end, I'd use an exact
multiple of half wave of better coax... RG-8 or 9913 or something.

73,
Dan