How to Calculate Values for dBm to Mangnitude.

Good Morning Sir / Madam.

    I am working on AD9371 Transceiver Board. i am implementing our own AGC for maintaining the Audo speech signal. in that context i am taking input from square root of i^2 + q^2 . and  taking one fixed threshold . how to convert my threshold level to magnitude. because i want to compare both input and my threshold these two are same units at that time only getting appropriate attenuation data. so how to do.

  i was seen in ILA at -40dBm level shows squared Magnitude as 3342. how to get this one. My ADC peak 707mv.

 please give me caluculation for convert dBm to Magnitude.

Thanks & Regards

P Prasanth