0% to 15% is quite normal, taking into account the prevailing weather.
Wood is naturally damp, it is affected by relative humidity and will be "wetter" in damp weather, dryer in dry weather.
15% to 20% may require further investigation, when compared to the average of all the other readings, re check the area after 3 months.
20% to 30% indicate areas where remedial work will need to be done.
Again recheck after 3 monts
30% and above,serious water penetration with inevitable deterioration and possible structural damage