I'm looking for a stable way to create a print composer template which automatically adjust its size to the area of interest (defined by a layer "perimeter" containing one polygon feature) at fixed scale 1:25'000.
I tried using the Atlas tools and set it to fixed scale. This works, but it's not stable and difficult to handle with different projects (different perimeter layers).
It most often does not render the map when Atlas is set to fixed scale in "controlled by Atlas", which means changing to "margin around feature". Further, it always kicks my preset scale and uses "nan", and I haven't found the logic to recreate it.