Hi, leommarisio, and welcome. Your English is excellent. How to know the ABV of a mead after you dilute the mead with water (or tea)? Simple. If you know the amount of honey you started with and the starting volume of the mead (in imperial measurements - say 3 lbs of honey to make 1 US gallon of mead) and if you know the real or the approximate specific gravity of the honey - say, about 1.035 for each pound (about 1.040 for each 1/2 kilo for 4 liters) or 13.5 % ABV and you then double the volume (making, in this example 8 liters) then you have halved the ABV - so about 7%. If you increased the volume by say 2 L (in this example) then the ABV would have been reduce to about 10% . Very approximately, 125 g honey in 1 L = a gravity of about 1.040 - double the amount of honey and you double the ABV , double the volume of the liquid and you half the ABV.

Hope this helps.