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The problem here is that pseudolength toaddrhpos is set in NF.lco to a negative value of -10mm with command \@setplength{toaddrhpos}{-10mm}.

That means that the value of 10mm is not the value between left paper side and address field, it is the value between the right paper side and the adress box. Usually the adress box is printed first (on the left side of the paper) and the location box follows then. With file NF-lco that changed as you already showed in your picture in your question.

sigindent is always calculated from the left side of the typing area, so you have to calculate the needed value for sigindent by yourself.

One possibility is:


That means we calculate the \paperwidth minus toaddrhpos (because its value is already minus we have to use plus here) minus toaddrwidth minus refhpos minus two times toaddrindent.

With that calculation in your corrected given code





% from name
\setkomavar{fromname}{Chef Abraracoursix}

\setkomavar{signature}{\noindent Chef Abraracoursix,\\Un chef à toujours bras long.}
\setplength{sigindent}{\paperwidth+\useplength{toaddrhpos}-\useplength{toaddrwidth}-\useplength{toaddrindent}-\useplength{toaddrindent}-\useplength{refhpos}} % <===============

% from adress
\setkomavar{fromaddress}{Place du village,\\Erqy,\\Armorique}

% place and location
\setkomavar{placeseparator}{, le }

% other komavar


% firsthead


  pdfauthor={Chef Abraracoursix},
  pdftitle={Invitation pour une banquet au sanglier Roti},
  pdfcreator={Emacs 26.1 (Org mode 9.1.9)}, 


\setkomavar{subject}{Invitation pour une banquet au sanglier Roti}

    Capitaine Paulus Romanicus\\
    Camp de Babaorum

\opening{Le Romain,}

Dans le cadre des rapports de bon voisinage et de notre hospitalité 
légendaire, nous vous invitons à festoyer avec nous sous le grand chêne, 
du coucher du soleil à l'aube, à la prochaine plein lune pour le mariage 
de la belle Fabala et de vigourix.
\closing{À la tienne !}



you get the following result:


It seems there is a difference of 5pt too much I do not know where it comes from (rounding errors, an indent I did not saw?). In the case of this mwe you can add them to the calculation with: