[Tex/LaTex] Tikz: drawing in perspective

calculationstikz-pgf

I am working on drawing a beam in perspective. I have my point I want to use as the perspective point (10, 3). What would be the way to draw along this line? For instance, consider

\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}
\draw (0, 0) rectangle (1cm, 1cm);
% I want to draw a 2cm line from (1cm, 0) along the ray that goes to my vanishing point
\end{tikzpicture}
\end{document}

1. I could set \pgfmathsetmacro\myangle{atan(3/10)} and use the \usetikzlibrary{calc} to issue \draw (1cm, 0) -- ++(\myangle:2cm);
2. I have tried issuing \def\myangle{tan(3/10)} but this doesn't work at all.
3. Or I could use scope and rotate a the line \draw (1cm, 0) -- (2cm, 0); by the intended angle.

After fixing tan to atan @PeterGrill, I realized there is a problem with drawing to the vanishing point. Only (0, 0) would be on the line with angle atan(3/10). How can make adjustments for the other points? I know I could figure out the math but can LaTeX handle this instead?

\usetikzlibrary{calc}
\draw (1,0) -- ($(1,0)!2cm!(vanishingpoint)$);