Cec,

The meter indicates SWR on the 50-ohm coax between meter and
transmitter. It HAS to be 50-ohm coax. Any other impedance and you
get the wrong answer. Without measurement of Zo = 50 ohms it can only
be assumed.

You then include in the calculation the measurement or assumption of
the Zo of the 50-ohm coax, and the measurement or assumption of Zo of
the twin-line, and the forward and reverse powers, and the SWR on the
twin line can be deduced or assumed.

But if you think you are measuring SWR on anything you are cheating
and fooling yourself.

In your particular case an assumptiom of Zo = 450 ohms for the
twinline would be very much in error because both you and I know you
have measured Zo to be 380 ohms. I can tell you what the SWR is on
YOUR feedline without getting out of this armchair. I don't need to
know your meter readings. ;o)
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Reg.