Document magic numbers

There are some 'magic' numbers in the code.
These should be extracted as constants, and documented.

In some cases the numbers appear to be arbitrary, in which case maybe they should be configurable.

No files, tests, or entry points are named. Start by locating numeric literals in the Python code and inventorying which are unexplained or repeated; then determine which values need constants, documentation, or configuration. Done means the scope and intended treatment of each identified number are agreed and covered by the relevant project checks.

Written by the indexing model from the issue text.