Then you need to consult
friction loss tables, along with your flow data, and establish the proper plumbing for the connecting line. Total pressure loss in the interconnect should only be one or two psi, in order for it not to be getting in the way of system performance.
As for your un-named backflow preventer, I will once again stress for the wider audience that any city water interconnect to ground water has to be done right. In fact, some localities flat-out prohibit the practice, with the laws squarely on their side, because a homeowner can have his Certificate of Occupancy revoked, and his house subject to being emptied of its occupants and the doors padlocked, until the offending plumbing is remedied and inspected, and the CO restored.