Could you do and example of this so I can understand it?
78
I want to know what 78km is in miles. Half of 78 is 39. A tenth of 78 is 7.8. 39+7.8 is
46.8 miles (Real answer is 48.4, so it's OK but not great. All I've done is multiplied it by 0.6. It's not hard to add a the missing bit but I won't do that here)
I want to know what 78miles is in km. Do the above and get 46.8. Now add the original 78.
124.8 km. (Real answer is 125.5. Not bad. All I've done is multiplied it by 1.6).
Maybe I have a weird brain but I find this really easy. It comes from years of running 10ks and half marathons, and riding 100km and 100 mile rides and thinking about such things to pass the time. Other people have their own pet methods and prefer them. I'm not saying this is a good method or that others should use it. It just happens to be what I do. If you don't like it, fair enough, I'm not asking you to.