Shopping Mall Water Tank Sizing

In the design of water tank capacity, normally there are two methods, which are volume estimated by quantity of sanitary fittings or occupancy. For commercial building such as shopping mall, I found that sizing by both of these methods will have a big different of water tank capacity because volume due to the sanitary fittings at the fewer toilets would be greatly small if compared to the high occupancy for the whole mall.

In term of sizing the shopping mall, what shall be the correct method ? Please advise. Thanks

Let's think worst case: Speak to the fire department or Marshal. If you can handle a fire you can probably handle the flush.

Ask the insurance company that issues the fire coverage if there is a difference in the premium based upon the available pressure or volume. The thinking here is that if you can do something to lower premiums you have some economic justification for your choice. Also determine if you can rent space on top for antennas and sell the ad space on the thing. These are the kind of things that I would want to know if I was paying for a water tower.