Beacon distance

Hello Good day, I had seen many questions about Indoor SDK for android and its clear that it wont be ready any time soon, so I’m trying to make my own estimation of the inside user position, knowing the GPS position of each beacon and making an estimation knowing the distance between the user and the beacon(s) it should be can of easy get a more accurate position of the user

so my question is whats the best way to know the distance between the user and the beacons? does the beacon gives that value? I had seen someone talking about the signal strength, if you could offer me the formular that you use to estimate the value I would really appreciate it, I dont know whats the exact relation between intensity and whats his distance equivalent and I dont think that it is a lineal relationship, so if you could give me that formula I would really appreciate it, Thank you