How do I calculate closed loop gain's deviation?
Assume loop gain \$k=A\beta\$. Then:

$$\begin{align*}
A_{CL}&amp;=\frac{1}{\beta}\cdot\frac{k}{1+k}
\end{align*}$$

\$\frac{1}{\beta}\$ is the ideal gain as \$A_{OL}\rightarrow \infty\$, so this means that the second factor would ideally be exactly 1. The error term is then the difference between the second factor and 1:

$$\begin{align*}
\bigg|1-\frac{k}{1+k}\bigg| \le 0.001\\\\
\bigg|\frac{1}{1+k}\bigg| \le 0.001\\\\
\big|1+k\big| \ge 1000\\\\
\big|k\big| \ge 999\\\\
\big|A\beta\big| \ge 999\\\\
\end{align*}$$