Hi Ali,
"I taking the full power value of the band for instance 20dB and then if I choose -3dB in Ruckus that would change to 10dB and if I want 4dB I can do -7dB approximately?"
If this is your question ? (Correcting the dBms to dB.)
You are correct, lets assume that when Tx power is set to auto/full it is 20dB, when you reduce it to -3dB then the power would be half'd which is 10dB, so if you want 4dB then it is -7dB by rules of 10's & 3's (-10+3 = 20 dB/10 = 2dB * 2 = 4dB).
Regards,
Abilash PR.