Centering text within custom shape

Question

On page 628 of the TikZ manual, it describes how to code the text anchor for a custom shape so that the label text is placed with its centre at the origin. I've done this to create a simple shape, which draws a square of side length 10pt. The text label should then be centered at the origin.

However, it doesn't seem to work correctly. Here's a screenshot:

On the left is a node defined with my new shape, with label \fbox{$f$}. On the right is a standard TikZ node with the same label, created with the draw option and inner sep=1pt. Both nodes are positioned with the same y coordinate - but clearly, the label for my shape hasn't been placed at the same height, and it is clearly not correctly centred in its box.

Question: how can I get the text label for a custom shape properly centred at the origin?

Here's the code.

\documentclass{article}
\usepackage{tikz}
\makeatletter
\pgfdeclareshape{myshape}
{
    \savedanchor\centerpoint
    {
        \pgf@x=0pt
        \pgf@y=0pt
    }
    \anchor{center}{\centerpoint}
    \anchorborder{\centerpoint}
    \savedanchor{\upperrightcorner}
    {
        \pgf@y=.5\ht\pgfnodeparttextbox
        \pgf@x=.5\wd\pgfnodeparttextbox
    }
    \anchor{text}
    {
        \upperrightcorner
        \pgf@x=-\pgf@x
        \pgf@y=-\pgf@y
    }
    \backgroundpath
    {
        \pgfsetstrokecolor{black}
        \pgfpathmoveto{\pgfpoint{-5pt}{-5pt}}
        \pgfpathlineto{\pgfpoint{-5pt}{5pt}}
        \pgfpathlineto{\pgfpoint{5pt}{5pt}}
        \pgfpathlineto{\pgfpoint{5pt}{-5pt}}
        \pgfpathclose
        \pgfusepath{stroke}
    }
}
\makeatother
\begin{document}
\setlength\fboxsep{0pt}
\begin{tikzpicture}
\node [myshape] at (0,0) {\fbox{$f$}};
\node [draw, inner sep=1pt, red] at (12pt,0) {\fbox{$f$}};
\end{tikzpicture}
\end{document}

Answer 1

\newanswer{comment above}

The apparent problem seems to be the calculation of the center that is independent from the text depth. Including the code given in the manual p.626 is enough for Jamie to carry on from there.

The proposed change is

\savedanchor{\upperrightcorner}
{
    \pgf@y=.5\ht\pgfnodeparttextbox
    \pgf@x=.5\wd\pgfnodeparttextbox
    \advance\pgf@y by -.5\dp\pgfnodeparttextbox
}