Place objects by hand

[Noeck]

Hi,

I would like to move some objects by hand. This minimal example shows
the beginning of a piano staff.

\version "2.18.2"

\new PianoStaff <<
  \new Staff { g-_ }
  \new Dynamics {
    %\override DynamicText.extra-offset = #'(-4 . 0)
    s\f
  }
  \new Staff { \clef bass g'-_ }
>>

The forte symbol is in between the two notes and prevents the staffs to
get closer together. extra-offset does not help as it comes too late.
How can I place the DynamicText such that the staffs can come closer
together?

Joram