Lots of great tips so far. Basing this on some of those and the biggest question is how much room you have in your horizontal.
Based on your original photo, with the yellow in my diagram being your existing salt cell and pipe, how much room do you have? If you are at roughly 20, you should be okay.
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You'll want to cut the yellow near the blue diverter, but leave enough room for your Tee fitting (the flow sensor that came with the RJ.). 2 inches should be minimum.
You'll want to cut the yellow near the red elbow, but leave enough room for your union - including sliding the rotating ring back. I'd say about 2.5 inches.
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Then you can plug in your RJ. as such....
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If you don't think you have enough room, you can put the flow switch before (below) the red elbow.