I would have to agree with rchahboune. A shopping centre is one of the most challenging environments, because (1) the space is open, so you need half as many access points because the RF propagation is so good (2) Where you can actually mount access points is key to getting the coverage you need.
For example, in an office setting, you can place an access point every 50 to 60 feet. In a mall, the space is open, so you can sometimes get away with 90 to 100 feet. If you deploy at 50 to 60 feet, the APs can see each other very well, so it will create tons of co-channel interference. In addition, the propagation will differ based on how high you mount the access points. You should try surveying with actual access points at every 90 feet at 12dbm and see what type of coverage you get. You would probably need a second pass to determine where you can ACTUALLY deploy access points to get the coverage you expect. You also need to account for all types of clients that would be deployed like handheld scanners, phones, laptops, etc to make sure they work at those distances.