First note that devices in sleep mode (screen off) might only send beacon requests every 10 minutes - so don't expect to get minute by minute data for each device - no matter which method you use.
To get an RSSI from the Meraki number in dBm:
RSSI - 95 = signal strength in dBm
It's hard to find, but buried deep in this document:
https://documentation.meraki.com/MR/Monitoring_and_Reporting/Location_Analytics
Personally, I would just create my own table of measurements relating the Meraki returned RSSI to distance.