Well, start measuring.
Read signal strength, 1m distance - write down the number, 2m distance - write down the number, etc, and you have yourself a calibration table.
Do make sure no-one walks in the way of your signal, the humidity of the air remains constant, that you don't get stray reflections that appear to be part of the signal or even be the signal, and that you definitely don't put walls or door s in the way. All of those will mess up your signal strength big time.
And hope you learned one important lesson: think before you build. Make sure your sensors are suitable for what they have to sense.