How to calculate dB of a transmitted power
If you have a system that causes 80% of its input power to be lost, then 20% of the input power will remain at the output. The power gain of the network is therefore:

$$
G = \frac{P_{out}}{P_{in}} =0.2
$$
$$
G|_{dB} = 10 \log_{10}(G) = 10 \log_{10}(0.2) \approx -7 \text{ dB}
$$