You are missing the point, yes i know mental maths or what ever you choose to call it does help but you need to learn to cheat. Near enough is good enough and all that. During your trainning you will come accross many rules of thumb listen to those who tell you them and learn.
I was pretty darn good at mental arithmetic but with all the noise and flak it all goes out of the window. An E6b (crappy5) bezel on your watch can help during those CRAFT moments (can't remember a Firkin Thing).
Do a google search on 'rules of thumb' you should find loads of helpful stuff
www.google.com Oh and RELAX
