Plumbing For Cistern and Shallow Well Pump

I have been working on a project to reclaim an old cistern behind my home and have some plumbing questions to finish it up. It is an old concrete parged cistern that is approximately 4400 gallons and has a "filter"tank that feeds into it that is filled with rocks and sand. I installed all of my downspouts to the filter tank and the cistern is filling up. My goal was just to use it to water my garden, plants and yard. I installed a new shallow well pump with a pressure tank, new suction line with a foot valve from the cistern. Inside the house, I installed a check valve and then a shutoff ball valve on the outlet. I was just going to run this to a hose bib and be done with it but then I started thinking about supplying it to the main toilet in my house. I haven't started researching the municipal code on this yet but I wanted some opinions on this. The toilet and supply line to the existing hose bib are on the same supply at the "end of the line". Exclusive of "Code", I was wondering what is "safe".

Here's how it would be laid out:

Municipal water -->ball valve shutoff -->tap in from Cistern Pump-->toilet supply line-->hose bib

My thoughts were that I would isolate the municipal water with the ball valve and then the cistern would run to the toilet and hose bib via the pump. If the cistern was low, I could shut off the supply from the pump and open up the municipal water to the toilet and hose bib.

My question is: Is the ball valve adequate to isolate the cistern rainwater or do I absolutely need a backflow preventer such as a dual check valve or a testable double check valve? If so, where should it be installed? Before or after the municipal line ball valve?

Sorry about the long post. Any help would be appreciated.

Thanks,
Shawn

Some places will allow a double check or RPZ between the city supply and cistern pump. Some places will not allow any connection between a city supply and other water source. You will have to find out what is code in your area.