For myself (YMMV) its an old 80/20 rule, majority of the stuff you now whether you can "hack it" or you need a longer prep. 80 implies you will wing it 9 out of 10. Bigger more complicated stuff/project/assignments is that remaining 20%. I'm with @ekzyis you learn by doing, you gain experience, nothing can beat that in my book. It applies in my opinion to most things in life coding or writing notwithstanding.