Then, I decided that 12pts font was too big and reduced it to 11pts. The spacements between equations need to be adjusted, and I guess that I shouldn't have used commands like \\[12pt] and \\[18pt].

Here's a MWE example showing the issue:

\documentclass[11pt,letterpaper,twoside]{book} \usepackage{lmodern} \usepackage{amsmath} \begin{document} Blablabla: \begin{gather*} x^2 - 3 y = 2, \\[12pt] \frac{1}{2} \, x y - z^3 = 2, \\[12pt] \frac{3}{4} \, x - y = z^2. \\[12pt] z + x - y = 1. \\[12pt] z^2 - y^2 = 0. \end{gather*} Blabla \begin{align*} x+ 3 y &= 2, \\[12pt] \frac{1}{2} \, x^2 - z &= 2, \\[12pt] \frac{2}{5} \, z + z^2 &= y. \\[12pt] z + x - y &= 1. \\[12pt] x^2 - y^2 &= -1. \end{align*} \end{document}

I want all the equations to be equally spaced, no variable vertical spacings.

What should be the proper commands to vertically dispose the equations, in a font size independant way, and so that the spacings stay constant for any set of equations in any environment (array, align, aligned, gather, gathered, ...)?