A ratio of amplitude quantities $A$ and $B$ can be expressed in decibels as:

$$20 \log_{10}\left(\frac{A}{B}\right) \text{ dB}.$$

A ratio of power quantities $A^2$ and $B^2$ can be expressed in decibels as:

$$10 \log_{10}\left(\frac{A^2}{B^2}\right) \text{ dB}.$$

The ratio of the squares of the errors of approximating an amplitude quantity $C$ by two approximations $A$ and $B$ can be expressed in decibels as:

$$10 \log_{10}\left(\frac{(A - C)^2}{(B - C)^2}\right)\text{ dB},$$

How should the ratio of the squares of the errors of approximating a power quantity $C^2$ by two approximations $A^2$ and $B^2$ be expressed in decibels?

$$10 \log_{10}\left(\frac{(A^2 - C^2)^2}{(B^2 - C^2)^2}\right)\text{ dB}$$

or perhaps:

$$5 \log_{10}\left(\frac{(A^2 - C^2)^2}{(B^2 - C^2)^2}\right)\text{ dB}?$$

Or is there no common convention?