The placement of the backflow assembly would depend on the application of the PRV. Remember that you will lose approximately 12psi through the backflow assembly.
Example 1: If the PRV is required to bring the total building pressure down to less than 80 PSI (PRV located at the water meter) there would be no problem locating the RPZ after the PRV
Example 2: If the PRV is being used to reduce the existing water pressure for a specific plumbing fixture i.e. a boiler make-up and the water pressure to that plumbing fixture requires a critical PSI range you would be better served to install the RPZ on the supply side of the PRV as the PRV then will further reduce the pressure to the required pressure range.
State of Illinois Plumbing code does not specify one way or the other.
Sorry for the ambiguous answer but it really does matter what the PRV application is for.
Mike Eisenhauer
ISAWWA Backflow Committee
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