Q:

Decreasing interline spacing in an align environment - within a node command

I am trying to put the following display in the contents of a node command in TikZ.
\begin{align*}
&\text{Present edge label of} \\
&\quad 2\bigl[(b-n)+(b-(n+1))\bigr] \\
&\qquad=2^{2}b-(2n+1)2
\end{align*}

The interline spacing is too big - for my liking. I tried using [0.25ex] and [0.5ex], but they are ignored. If the align* environment does not allow for decreasing interline spacing, I would appreciate an alternative suggestion for getting a similar display with regards to horizontal positioning but with smaller interline spacing.
Here is the code for the TikZ diagram.
\documentclass{amsart}
\usepackage{amssymb}
\usepackage{mathtools,array}

\usepackage{tikz}
\usetikzlibrary{calc,intersections}

\begin{document}

\begin{tikzpicture}[nodes={inner sep=0, font=\scriptsize,
execute at begin node={\setlength\abovedisplayskip{0.75ex}%
\setlength\belowdisplayskip{0.5ex}%
\setlength\abovedisplayshortskip{0.75ex}%
\setlength\belowdisplayshortskip{0.5ex}}},
shorten/.style={shorten >=#1,shorten <=#1}]

%A sequence of graphs is drawn, starting with the vertex with the b-label b.

%Here is the blow-up of the vertex labeled b.
\draw[fill] (-4,0) circle (1.5pt);
\node[anchor=north] (label_for_Vertex_b) at ($(-4,0) +(0,-0.25)$){\textit{b}};
\node[anchor=south] at ($(-4,0) +(0,0.25)$){$\dfrac{0}{1}$};
%
%
\draw (-4,-3) -- (-2,-3);
\draw[fill] (-4,-3) circle (1.5pt);
\draw[fill] (-2,-3) circle (1.5pt);
%
\node[anchor=north] at ($(-4,-3) +(0,-0.25)$){\textit{b}};
\node[anchor=south] (label_for_Farey_Fraction_at_Vertex_b) at ($(-4,-3) +(0,0.25)$){$\dfrac{0}{1}$};
%
\node[anchor=north] (label_for_Vertex_b-1) at ($(-2,-3) +(0,-0.25)$){$b - 1$};
\node[anchor=south] at ($(-2,-3) +(0,0.25)$){$\dfrac{1}{1}$};
%
%
%An arrow is drawn to the next diagram.
\draw[-latex, line width=0.8pt, shorten=7.5pt] (label_for_Vertex_b) to[bend right=30] node[midway, left=1.5mm, align=center]
{Blow-up of\\vertex \textit{b}} (label_for_Farey_Fraction_at_Vertex_b);

%Here is the blow-up of the vertex labeled b-1.
\draw (-4,-6) -- (-2,-6) -- (0,-6);
\draw[fill] (-4,-6) circle (1.5pt);
\draw[fill] (-2,-6) circle (1.5pt);
\draw[fill] (0,-6) circle (1.5pt);
%
\node[anchor=north] at ($(-4,-6) +(0,-0.25)$){\textit{b}};
\node[anchor=south] at ($(-4,-6) +(0,0.25)$){$\dfrac{0}{1}$};
%
\node[anchor=north] at ($(-2,-6) +(0,-0.25)$){$b-1$};
\node[anchor=south] (label_for_Farey_Fraction_at_Vertex_b-1) at ($(-2,-6) +(0,0.25)$){$\dfrac{1}{1}$};
%
\node[anchor=north] at ($(0,-6) +(0,-0.25)$){$b-2$};
\node[anchor=south] at ($(0,-6) +(0,0.25)$){$\dfrac{2}{1}$};
%
%
\draw[-latex, line width=0.8pt, shorten=7.5pt] (label_for_Vertex_b-1) to[bend right=30] node[midway, left=1.5mm, align=center]
{Blow-up of\\vertex $b - 1$} (label_for_Farey_Fraction_at_Vertex_b-1);

%Here is the blow-up of the vertex labeled b-n.
\draw (-4,-9) -- (-2,-9) -- (0,-9) (2,-9) -- (5,-9);
\draw[fill] (-4,-9) circle (1.5pt);
\draw[fill] (-2,-9) circle (1.5pt);
\draw[fill] (0,-9) circle (1.5pt);
\node at (1,-9){$\ldots$};
\draw[fill] (2,-9) circle (1.5pt);
\draw[fill] (5,-9) circle (1.5pt);
%
\node[anchor=north] at ($(-4,-9) +(0,-0.25)$){\textit{b}};
\node[anchor=south] at ($(-4,-9) +(0,0.25)$){$\dfrac{0}{1}$};
%
\node[anchor=north] at ($(-2,-9) +(0,-0.25)$){$b-1$};
\node[anchor=south] at ($(-2,-9) +(0,0.25)$){$\dfrac{1}{1}$};
%
\node[anchor=north] at ($(0,-9) +(0,-0.25)$){$b-2$};
\node[anchor=south] at ($(0,-9) +(0,0.25)$){$\dfrac{2}{1}$};
%
\node[anchor=north] at ($(2,-9) +(0,-0.25)$){\textit{b-n}};
\node[anchor=south] at ($(2,-9) +(0,0.25)$){$\dfrac{n}{1}$};
%
\node[anchor=north] at ($(5,-9) +(0,-0.25)$){$b-(n+1)$};
\node[anchor=south] at ($(5,-9) +(0,0.25)$){$\dfrac{n+1}{1}$};
%
%
%A "pin" is drawn between the midpoint of last two vertices and the label of the mediants of these vertices.
\draw[-latex, dashed, line width=0.8pt, shorten <=3mm, shorten >=1mm] ($(3.5,-9) +(60:2)$) -- (3.5,-9);
\path node[anchor=south, align=center,text width={width("future vertex")}]
at ($(3.5,-9) +(60:2)$){future vertex\\of\[\dfrac{2n+1}{2}\]};
%
%A "pin" is drawn between the midpoint of the edge between the last two vertices and its label.
\coordinate (label_for_Edge) at ($(3.5,-9.5) +(0,-0.75)$);
\draw[draw=gray, line width=0.8pt, shorten <=1mm, shorten >=1mm] (3.5,-9) -- (label_for_Edge);
\node[anchor=north, align=center, inner sep=0, font=\scriptsize] at (label_for_Edge)
{$\begin{aligned} &\text{Present edge label of} \\[0.5ex]
&\quad 2\bigl[(b-n)+(b-(n+1))\bigr] \\[0.25ex]
&\qquad=2^{2}b-(2n+1)2
\end{aligned}$};

\end{tikzpicture}

\end{document}

A:

The align* and aligned environments both allow for decreasing interline spacing. When adding a length after the \\ command to add spacing between two lines, this length accounts for an additional interline spacing, positive or negative. 
If I compile your example, the commands [0.25ex] and [0.5ex] are not ignored, they just add more spacing. If you want the interline spacing to be decreased, you have to insert a negative length there. Using your code, the interline spacing in the aligned environment is decreased if I replace the last node with
\begin{aligned} &\text{Present edge label of} \\[-1.5ex]
&\quad 2\bigl[(b-n)+(b-(n+1))\bigr] \\[-1.5ex]
&\qquad=2^{2}b-(2n+1)2
\end{aligned}

