Nice layouts!

First off it appears, based on your description of where the test hose bib was located, that it was the one shown on the far left of your dwg. The other being taken off right after the Water Treatment System(WTS). **When you have time the hose bib at the WTS shown be tested at various flow. **Wouldn't matter where the flows were taken but would be beneficial to know the flow rate corresponding to the pressure, if you could manage that. Just another piece to the puzzle.

Looking at what you had already stated on pressure readings as you added flows from various locations, I approximated the flow rates based on the types of fixtures you used and approximated pipe lengths rather on the long side, for the purpose of estimated pressure losses.

Findings:

For the first flow of the Kitchen sink shows the pressure loss through the 3/4" from the WTS to the point where it tees off with the 1/2" line. I assumed 50 feet of 3/4" pipe at a 4 gpm flow. That only amounts to approx. 1.25 psi pressure loss. So that 10 psi loss your gauge showed included the added losses of the WTS. About 9 psi.

When I looked at adding the 2nd flow(say about 3 gpm from the 1st floor bathroom) and added pipe length losses due to the added 1/2" piping it totaled about 3 or 4 psi, while your gauge had shown an additional 15 psi. So at the slightly higher flow rate the WTS accounted for an **additional** 11 or 12 psi losses. I have to run right now but you get the picture so far?

I returned...So with that 3rd flow added(say about 3 gpm for that upstairs sink) that could account for friction loss up to about 10 psi or so depending on pipe lengths, while again the gauge went down another 15 psi(didn't travel through any additional piping). Only added flow to a section of the 1/2" and a section of the 3/4".

So roughly speaking the WTS accounted for about a total of 30 psi at the total flow rate of about 9 to 10 gpm.

Putting a gauge on that other hose bib would be a much better indication of actual pressure loss through that WTS at a given flow. Little to no piping involved for friction loss, between WTS and tee for hose bib pipe.