Just my .02...
For any SWR meter that rectifies RF through a diode (SWR meter in a radio, most external SWR meters) to be accurate, it needs to be directly connected to the load/antenna or use a 1/2 wave length jumper connected to the load/antenna to be accurate.
Why: Because the RF going through the coax will be at the same potential on each end of a 1/2 wavelength of coax (or multiple of). Therefore, when the RF is rectified, it will give the same reading on the SWR meter. This is the reason you will see different SWR readings when you change your jumper to a longer or shorter one or a different efficiency when there is reactance in the load.
IMHO, to properly "tune" your antenna, you can use a meter or analyzer to get a "ballpark" tune by connecting directly to the antenna or through a 1/2 wavelength coax.
The "final" tune should always be with the amp on and at full output with everything in place and how you plan on running things.
Since your Carl Built doesn't have a SWR meter in it, use a double male pl-259 and attach a SWR meter directly to the output connector. Connect a 1/2 wavelength coax or multiple of (what ever it takes to reach the antenna). Make sure the SWR meter can handle the output of the Carl Built. Tune antenna for lowest reflect on the meter.
I prefer this kind of SWR meter as it uses a straight through design (uses inductive coupling) and has minimal impact when placed in line. They were all made in the same Japanese factory - just had different names put on them. They are good for 1KW. Also recommend disconnecting the 2 diodes on the FS connection so stray RF doesn't come in on it.
https://www.ebay.com/itm/165262121844?hash=item267a63db74:g:LBIAAOSwmSlh0Gcq
https://www.ebay.com/itm/255318322901?epid=778359977&hash=item3b72286ed5:g:Ys0AAOSw-2thXxu4
https://www.ebay.com/itm/144323966329?epid=2254344538&hash=item219a60f579:g:sREAAOSweJ9hsVo6
If you are going to run a SWR meter permanently, try finding one that has a remote sense that can handle 1KW and connect the remote directly to the output of the Carl Built.
For adjusting the input SWR on the Carl Built, that is done internally in the box. The pic is the input/output relay in the 400HD. It is not an RF relay, it is a power relay. It is not deigned to maintain a 50 ohm impedance between its contacts.
The lower green wire is the input. I see a single Silver Mica to ground (to the right of the input wire). If it is built right, that will be from the N/O relay contact to ground. You can adjust that value to change your input SWR when the box is ON. You will see a very short wire that goes between the 2 N/C relay contacts on the top of the relay. You can add capacitance (picofarads) to circuit ground there to adjust your input SWR when the box is OFF.
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Good Luck!
