Bluetooth RSSI to actual distance

I have recently been investigating the new bluetooth proximity exchange profile. As part of a new library I am building I needed to map RSSI values to real world distances. Understanding that RSSI can be effected by local obstructions I attempted to create a clear environment for testing.

The set up is as follows. I begin by building 3 bluetooth beacons using both Raspberry models and an iPhone5 along with iPhone app that would track them. I then created a large empty area to place my beacons in. I then took RSSI measurements using the latest iPhone5S at 1 foot intervals to 10 feet away then 2 foot intervals from 10ft to 20 ft. I then graphed the results.