Table wanted that breaks across pages and has the wanted horizontal size

Question

Working quite a time now with tables I get the impression that sizes are not respected.

I usually do:

  \begin{longtable}[H]{|m{0.3\linewidth}|m{0.7\linewidth}|}
    \hline
    \textbf{Header1} & \textbf{Header2}\\\hline
    \endfirsthead
    \hline
    \textbf{Header1} & \textbf{Header2}\\\hline
    \endhead
    Content\\\hline
    \caption{Caption}
  \end{longtable}

The horizontal size is bigger than expected. I compare the endpoint of the line above the footer with the right margin of the table. Those 2 shall get the same x-value, which is not the case yet.

When I use tabularx the x-value is the same:

  \begin{tabularx}{\linewidth}{|lX|}
    \hline
    Content & Content \\
    \hline
  \end{tabularx}

But since I have tables that are longer than one page I need a new solution since tabularx just keeps continuing the table into the footer and does not break on a new page. As suggested here, I changed to longtable: Continuing table on multiple pages with tabularx

longtable properly breaks across pages (even repeating the header) but does not respect the size, tabularx respects the size but does not break across pages. Is there a working solution to this issue?

EDIT: For all those searching the answer here is the working code I use now (2 columns example):

\begin{longtable}{|m{0.25\dimexpr\linewidth-4\tabcolsep-3\arrayrulewidth}|m{0.75\dimexpr\linewidth-4\tabcolsep-3\arrayrulewidth}|}
    \hline
    \textbf{1} & \textbf{2}\\\hline
    \endfirsthead
    \hline
    \textbf{1} & \textbf{2}\\\hline
    \endhead
    1 & 2\\\hline
    \caption{1}
  \end{longtable}

Answer 1

You have to leave space for the column padding and rules so

\newlength\mylength
\setlength\mylength{\dimexpr\linewidth-4\tabcolsep-3\arrayrulewidth}
 % in general you need to remove
 % 2\tabcolsep for each column and 1\arrayrulewidth for each |

....

\begin{longtable}{|m{0.3\mywidth}|m{0.7\mywidth}|}