How to solve the 10.09999 rounding problem with pgfmath?

Question

I'm trying to create a technical drawing of a magnetic write head with dimensions on it. I'm using some macros which help a lot from here: Dimensioning a technical drawing.

\documentclass{beamer}

\usepackage{tikz}
\usetikzlibrary{positioning,arrows,shapes.arrows,shapes.geometric,calc,decorations.markings}
\usepackage{color}

\pgfarrowsdeclarecombine{|<}{>|}{|}{|}{latex}{latex}
\def\Dimline[#1][#2][#3]{
    \begin{scope}[>=latex] % redef arrow for dimension lines
        \draw let \p1=#1, \p2=#2, \n0={round(veclen(\x2-\x1,\y2-\y1))} in [|<->|,
        decoration={markings, % switch on markings
                mark=at position .5 with {\node[#3,scale=0.6] at (0,0) {\DimScale{\n0}};},
        },
        postaction=decorate] #1 -- #2;
    \end{scope}
}
%% The following macro is used to scale a dimension from points to the
%% display scale.  The following code divides the number of points by
%% 28.4 to roughly get the width in centimeters (rounding to the
%% nearest millimeter):
\def\DimScale#1{\pgfmathparse{round(#1/28.4*100)/10}\pgfmathresult nm}

\newcommand{\headDesignOne}[3]{
    \draw[step=1cm,gray!50,very thin] (-10.5,-10.5) grid (10.5,10.5);
    \draw[->,thin](-10,0)--(10,0);\draw[->,thin](0,-10)--(0,10);
    \coordinate (yOffset) at (0,0);
    \coordinate (O) at (0,0);
    \draw[very thin,blue!30] (yOffset) circle (#3);

    % coordinates of the main pole
    \draw[fill=gray, fill opacity=0.5]
    ($(270-#1:#3)+(yOffset)$) coordinate (A) -- 
    ($(90+#2:#3)+(yOffset)$) coordinate (B) -- 
    ($(90-#2:#3)+(yOffset)$) coordinate (C) --
    ($(270+#1:#3)+(yOffset)$) coordinate (D)-- cycle;
    \Dimline[($(B)+(0,-0.5)$)][($(C)+(0,-0.5)$)][below];
    \Dimline[($(A)+(0,0.5)$)][($(D)+(0,0.5)$)][above];
    \Dimline[($(D)+(3.5,0)$)][($(D|-C)+(3.5,0)$)][right];\draw(D)--+(3.5,0);\draw(D|-C)--+(3.5,0);
}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Begin document           %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{document}
\begin{frame}{Conventional writer design 1}
  \begin{tikzpicture}[scale=0.4]
    \path[use as bounding box](-10,-10) rectangle (10,10);     % Set the BB
    \headDesignOne{20}{40}{5}
  \end{tikzpicture}
\end{frame}
\end{document}

Unfortunately I can't paste the result as I'm new, but when I typeset the above, one of the dimensions that comes out is 85.59999

I have 2 questions:

1. How do I make the 85.59999 to be 85.6?
2. How do i take the 85.6 or the 64.4 that was computed in the \Dimline function and re-use that number? For example I want to put that number in a table listing some of the key parameters for this design.

Answer 1

The problem is the inaccurate division step after the rounding operation, which is necessary because you can't specify the number of decimal digits for the rounding operation. A solution would be to use \pgfmathprintnumber[precision=1]{\pgfmathresult} to round and output the number. If you want to use the rounded number elsewhere, you can use \pgfmathprintnumberto{\pgfmathresult}{\roundednumber} to store the rounded number to a macro.

Changing the scaling function to

\def\DimScale#1{\pgfmathparse{#1/2.84}\pgfmathprintnumberto[precision=1]{\pgfmathresult}{\roundednumber} \roundednumber nm}

yields

Answer 2

siunitx has a \num function that you can use to round numbers. Some details here. For example, \num[round-mode=places,round-precision=1]{85.99999} should give you 85.9.

Or you can use \pgfmathprintnumber although this requires pgf 2.10. (Or at least, I can't get it to work with 2.00)