Day 048/16. I can still use it.....

Way back years ago I did my GCSE O Levels and Maths A level using one of these. I didn't get my first electronic calculator until after I left school to study Electronic and Electrical Engineering.
I was always taught to 'sense check' my calculations, something I still do to this day. Do a very rough calculation in your head to make sure the answer you have makes sense. Those lessons all those years ago still hold me in good stead and have saved me from some costly and embarrassing mistakes.