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As Miro has pointed out, tuning an antenna for resonance by looking for a pure resistance
in the nanovna display must be done when measuring at the antenna feedpoint.

This can be done by attaching your already calibrated VNA directly to the feedpoint.

Or you can perform the calibration procedure with the nanovna
in the shack, the Open-Short-Load devices at the antenna end of the coax.
This will allow the VNA to calibrate out the effects of the coax.

Once the antenna is tuned to be purely resistive, just hook up the coax and
use your antenna tuner in the shack to make the antenna system look like 50 ohms
to your transmitter.
There will be some reflections in the coax between antenna and antenna tuner
if the antenna is not exactly 50 ohms, these reflections will burn up some of the
transmitter's power as heat due to attenuation (mostly resistive losses in the
not-quite-ideal coax), but below 30 mhz with 100 feet of RG8 these losses
should be insignificant.
Some in the forum may disagree, feeling you should climb up on the roof and
twiddle knobs to tune your antenna whenever you move to the far end of the band.
But Maxwell disagrees, it's the one point that he keeps hammering away at
in Reflections III, and in his half dozen articles in QST around 1973.
Other than the losses in the non-ideal coax incurred by a few round trips that a
fraction of the transmitted power takes due to reflections, all of the transmitter's power
eventually goes out the antenna.

Antennas get really complicated when you dig deep into the details.
And are fodder for all sorts of arguments.

Jerry, KE7ER