If I’m not mistaken, the distance is in meters, and TxPower really means the power received at a 1 meter distance (i.e. the value received by an iPhone 5S at 1 meter.) For the old (and possibly newer) Estimote beacons, this related to your actual TX power (which can be +8 dBm as you have it) - 62. So for a +8 dBm tx power, the power received at 1m should be -54 dBm. Of course you should take the 1 meter power from the beacons actual message, because it will depend on the beacons antenna, case, etc, and this value could be calibrated by the manufacturer for your specific beacon.
Anyways (assuming I’m correct) the formulas should be:
RSSI = -20 * log10(distance in meters) + RssiAtOneMeter
distance in meters = pow(10, (RssiAtOneMeter - ReceivedRSSI) / 20)
And for the older gen Estimotes, in general:
RssiAtOneMeter = TxPower - 62
I realize this post is pretty old, but thought it might be nice to clarify for those that run across this. And if I’m mistaken, please correct me!