I'm not 100% sure what you mean by raw data, I'm assuming you mean when you look at clients in the dashboard it reports dB?
What I meant is modifying the minimum basic rate (MBR) aka Minimum bitrate, I personally start at 24Mbps but sometimes I look at increasing it. It will hopefully help with reducing people staying connected while they move away from the associated AP.
The MBR can be set on each SSID by going to Wireless | Configure | Access Control | Wireless options (it's at the bottom). Slide the bar to adjust the rate. Note it can be set using the RF Profiles but I personally like to do it per SSIDs (occasionally I come across a scenario where I need different MBRs for different SSIDs - usually when supporting VoIP).
Also, what do you have band selection (also under Wireless options)? Unless you can disable 2.4GHz, which being a Mall you probably can't, you'd probably want to make sure you're using band steering.
What power levels are the APs set at?
40 APs sounds like a lot, how big is this Mall? Using 2.4GHz will most likely give you a lot of co-channel interference (CCI). You'll also want to make sure your channel widths on 5GHz are narrow, i.e. 20MHz.