None of those applications are very good in translating world units into a scaled drawing.
Inkscape is great, but suitable for artistic drawings, not for technical drawings like this.
I learned QCAD for this purpose. (www.ribbonsoft.org)
. It will take you at most two hours to learn how to use it using the excellent tutorial it comes with.
The way of entering your drawing is completley different from Inkscape, but it makes sense for technical drawings. (It resembles the AutoCAD way)