On Tue, 26 Sep 2017, Jean-Marie Pacquet wrote:
My scrguien.pdf is old as well (file dated 2013-02-25). It has references to version 3.10 of KOMA-Script.
Jean-Marie, My KOMA-Script version is ancient: 2004; think the version is 2.98. I just downloaded the latest from CTAN (2017-09-07 but have totally forgotten how to upgrade an installed package. I'm running TeXLive-2017 and I _think_ that I cp the koma-script.tds.zip file to /usr/share/texmf/ and unzip it there, but I'm not confident that's the proper way.
These \@setplength should be in the lco file (letter class options) which is called in the Latex preamble with \LoadLetterOption{lcofile}% I don't think you can use them anywhere in your Lyx document. Have a look at any of the lco file you have (DIN.lco f.ex.).
I'll look for (and in) .lco files. First need to upgrade KOMA-Script. Many thanks, Rich