I would never do this much work in my head :-) You can get a decent answer just by glancing at the fixed scales of a slide rule [1]. This is also called a nomo

I would never do this much work in my head :-) You can get a decent answer just by glancing at the fixed scales of a slide rule [1]. This is also called a nomogram, and you can make your own custom ones programmatically with PyNomo [2]. [1] https://youtu.be/dT7bSn03lx0?t=11m45s https://youtu.be/dT7bSn03lx0?t=11m45s [2] http://pynomo.org/wiki/index.php/Main_Page http://pynomo.org/wiki/index.php/Main_Page