It is quite clever. My own rule of thumb is "add half of the amount in miles, then 10% of the (original) amount in miles". What's interesting is that it happens

It is quite clever. My own rule of thumb is "add half of the amount in miles, then 10% of the (original) amount in miles". What's interesting is that it happens to be just as precise as the method shown above (ie. 1.6 v 1.609 v 1.618 )