Let K = 10^(A_dB / 20) (Voltage ratio) or K_p = 10^(A_dB / 10) (Power ratio)
Pi Network (Zin = Zout = Z0):
R_shunt = Z0 * (K+1)/(K-1) ; R_series = Z0 * (K^2-1)/(2K)
Tee Network (Zin = Zout = Z0):
R_series = Z0 * (K-1)/(K+1) ; R_shunt = Z0 * (2K)/(K^2-1)