I'm trying to get the vertical spacing in some tables to be less squished and I thought I would do so by making the rows a bit taller and centring the text vertically in the rows. The code below uses the advice from the second answer in this question to add '[XXpt]' after the line break where xx specifies a line height in order to make one of the rows taller. In terms of centering the text vertically, my understanding is that the 'm' prefix in front of the row widths below should do that.
Based on my understanding, the first row of the table below should be tall and have vertically centred text but it is only tall with text at the top. Can anyone tell me why and how to fix it?
\documentclass{report}
\usepackage{supertabular}
\usepackage{hhline}
\usepackage{array}
\begin{document}
\begin{table}
\begin{center}
\begin{supertabular}{m{0.7059598in}m{0.45105985in}m{0.61285985in}m{0.02in}m{0.7573598in}m{1.0121598in}m{0.07535984in}m{0.45105985in}m{0.9045598in}}
\hhline{~--~--~--}
&
\multicolumn{2}{m{1.0893197in}}{Shouldn't these} &
&
\multicolumn{2}{m{1.907in}}{Headings be centered} &
&
\multicolumn{2}{m{1.4343598in}}{Vertically?}\\ [40pt]
\hhline{~--~--~--}
Phantom & Mean & St. Dev. & & Mean & St. Dev. & & Mean & 95\% CI\\\hline
Seroma 1 & 5.73 & 0.04 & & 5.57 & 0.14 & & 0.16 & [-0.06, 0.37]\\ [20pt]
Seroma 2 & 3.75 & 0.03 & & 3.61 & 0.13 & & 0.15 & [-0.08, 0.37]\\
\end{supertabular}
\end{center}
\end{table}
\end{document}

\\[xx]adds to the depth of the previous row, anmdoes not centre in the space for the surrounding row, it just puts the baseline of the content in the contents vertical centre (like\parbox[c]) finally tex is not accurate to more than about 5 dp , so 0.7059598in is a bit odd, also if you have a span it should be\dimexpr0.7059598in+0.45105985in+2\tabcolsep\relaxwhich is rather more than the 1.0893197in you have used. – David Carlisle Oct 28 '17 at 08:23supertabularin atableenvironment. – egreg Oct 28 '17 at 08:42