I'm trying to use a tabular with `\underbrace`s under some of the columns. I don't seem to have figured out the right dimensions of the column widths and/or the braces -- I naively thought that using `\multicol` might simplify the problem. I'd like the columns to center on the braces (or vice versa). Any suggestions gratefully received... (MacTeX 2017)

```
\documentclass{article}

\usepackage[showframe]{geometry}
\usepackage{calc}
\usepackage{xparse}
\usepackage{array}

\newlength{\entrywd}
\newlength{\entrybracewd}

\setlength{\tabcolsep}{6pt}

\newenvironment{testtable}[2][\textwidth]{%
    \setlength{\entrywd}{#1/#2-2\tabcolsep} 
    \setlength{\entrybracewd}{#1/#2-\tabcolsep} 
    \begingroup
        \centering
        \tabular{*{#2}{p{\entrywd}}}
}{%
        \endtabular
        \par
    \endgroup
}

\newcommand{\mkbrace}[1]{%
    \entryi[#1]{\raisebox{0.5\baselineskip}{$\underbrace{\hbox to #1\entrybracewd{\hfill}}$}}%
}

%% Used this because an optional argument of \newcommand does not work with \multicolumn:
%% https://tex.stackexchange.com/questions/257902/defining-multicolum-by-macro-with-optional-argument/257904#257904
%% https://tex.stackexchange.com/questions/17133/custom-macro-for-multicolumn-with-without-xparse

\NewExpandableDocumentCommand{\entry}{ O{1} m }{%
    \multicolumn{#1}{>{\centering}p{#1\entrywd}}{#2}%
}

\NewExpandableDocumentCommand{\entryi}{ O{1} m }{%
    \multicolumn{#1}{>{\centering}p{#1\entrybracewd}}{#2}%
}

\parindent0pt
\begin{document}

\begin{testtable}{7}
    \entry[2]{one and some more of this sort}&\entry{bla bla bla and more}&\entry{this goes on and on and more}&\entry{bunch of really boring stuff}&\entry{I wish I could figure out}&\entry{so that I could move on with this nonsense}\\
    \mkbrace{2}&\mkbrace{2}&\mkbrace{3}\\
\end{testtable}

\begin{testtable}{5}
    \entry[2]{one and some more of this sort}&\entry{bla bla bla and more}&\entry{this goes on and on and more}&\entry{bunch of really boring stuff}\\
    \mkbrace{2}&\mkbrace{2}&\mkbrace{1}\\
\end{testtable}

\begin{testtable}[0.5\textwidth]{4}
    \entry[2]{one and some more of this sort}&\entry{bla bla bla and more}&\entry{this goes on and on and more}\\[-\baselineskip]
    \mkbrace{2}&\mkbrace{2}\\
\end{testtable}

\end{document}

```

[![example of tabular with underbrace](https://i.stack.imgur.com/VxoIP.png)](https://i.stack.imgur.com/VxoIP.png)