questions about NR 1.6

[Federico Bruni]

Hi

I'm translating NR 1.6 and I've found a sentence which is not
straightforward:
http://lilypond.org/doc/v2.17/Documentation/notation/modifying-single-staves

"""
However, it is possible to set ledger line thickness independently of staff
lines. The two values required multiply the staff line thickness with the
staff line spacing and are then added together to give the ledger line
thickness.

\new Staff \with {
  \override StaffSymbol.thickness = #2
  \override StaffSymbol.ledger-line-thickness = #'(0.5 . 0.4)
}
{ f'4 a, a,, f }

"""

Can you tell me what the two values refer to and what's the final value of
ledger-line-thickness?

I'm writing to lilypond-bug in case you think it's worth changing the doc...

Thanks
Federico