How to fix the vertical spacing above and under equations?

Question

I'm having problems with vertical spacing above and under equations and I don't know what to do anymore. Here's a MWE that reproduces my problem (I wasn't able to make a simpler version in English, so sorry for the French stuff but that's irrelevant!):

\documentclass[11pt,letterpaper,twoside]{book}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[french]{babel}
\usepackage{lmodern}
\usepackage[total={6in,10in},left=1.5in,top=0.5in,includehead,includefoot]{geometry}
\usepackage[nodisplayskipstretch]{setspace}
\setstretch{1.1}
\raggedbottom
\usepackage{microtype}

\begin{document}

%\setlength{\abovedisplayskip}{11pt}
%\setlength{\abovedisplayshortskip}{0pt}
%\setlength{\belowdisplayskip}{\abovedisplayskip}
%\setlength{\belowdisplayshortskip}{\abovedisplayskip}
\setlength{\jot}{3ex}

Sans chercher à être rigoureux, il vaut la peine de montrer que l'égalité entre les deux potentiels a lieu sous la limite thermodynamique (nombre très grand).  Pour les systèmes possédant un très grand nombre de particules, la série doit être dominée par un terme beaucoup plus grand que les autres, que l'on peut supposer centré (ceci n'est justifié que si l'écart relatif est suffisamment faible).  Le maximum de la fonction
    \begin{equation}
        \mu = \frac{\partial \Phi}{\partial N} = -\, x.
    \end{equation}
est obtenu en annulant sa dérivée par rapport à la variable évaluée en $N = \bar{N}$ (cette variable discrète peut être considérée comme une variable continue pour ses très grandes valeurs).  Ceci donne
    \begin{equation}
        \mu = \frac{\partial \Phi}{\partial N} = -\, x.
    \end{equation}
Puisque le nombre est susceptible de fluctuer autour de sa valeur moyenne, on peut développer la fonction en une série de puissance.  Au second ordre, on obtient bla blabla blaaaaa!

\end{document}

Here's a preview of my vertical spacing problem:

As you can see on this picture, the vertical spacing above and under equations is variable. I hate that! I want a perfectly constant spacing for all equations, above and under the text. When I comment out the \setlength commands in this MWE, I could get a much better display, but still it isn't perfect. Depending on the text and equations, I may get some variable spacing. So I have two questions:

1. How can I absolutly fix the vertical spacing above and under each equation?

2. What should be a natural spacing value? I used 8pt in that MWE example, but it needs to be scalable if I decide to change the global font size. I don't know what should be the proper space.

Please, take note that I'm not asking about the shortskip of equation (2) in this example, which is fine. Just see the three thin vertical red lines I've draw on the picture.

Answer 1

With the new uploaded image the two lengths marked by the arrow differ because of user settings. The first is based on \belowdisplayskip and the second (as the line above the display is short) uses \belowdisplayshortskip. If you want the spaces to be equal just set these to the same value, eg

\setlength\belowdisplayshortskip{\belowdisplayskip}

would make them both use the value from the first equation.