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.
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.