Hot on the topic of sketching – ‘eyeballing’ is one of the essential traits needed to product good sketches. It’s the ability to accurately place where a point or a line should be by imaginarily-visualizing them: finding a midpoint; the bisecting angle; drawing parallel lines, etc. Without good ‘eyeballs’ it is horribly difficult to get an accurate sketch out.
So, test yourself right here with a series of exercises to determine how good your visualization skills really are (the lower the better).
I got a 3.82 (quite a few bad judgments on my second round 
– don’t lose concentration!):
