LOL....I like how you old farts all like to act like it is a bad thing that people don't waste time practicing things which are no longer really of much practical use.  I can do differential calculus with a pencil, but I can do it fast enough to actually be useful with a computer.   How much time do you think Deere technical schools spend teaching magnetos these days?

I figure in my head when it is advantageous and when an error is not critical.  If it matters, I am going to use a calculator or a computer and/or write it out so that I can check my work.  The only way to do that in your head is to repeat the problem several times, preferably in different manners, so that you can establish a confidence level in your accuracy.  If it is written down, at least you can check for errors.  

No disrespect intended...I'm rapidly becoming one of the old farts myself.