Imagine what it is like to have been taught and used imperial measurements throughout your Schooling, at University, during National Service, at the time of your wedding and birth of your son, and then to be told everything must change! Couple that with a brain which refuses to tackle mental arithmetic and you will see my problem, and why I always carry a 10ft, sorry 3 metre tape measure in my trouser pocket.
The whole point of the metric system is that it doesn't require mental arithmetic. If you can multiply by 10, you're all good.