Juniour - Will you take it from professional pilots who have been in the industry just about all their working lives that the point you are trying to make about calculations is not valid?
Calculations made by a pilot relating to the job are carefully completed, using tables, computers, paper and pen and then scrutinised by at least one other member of crew.
Calculations, done in the head, possibly in a bar, may well not be accurate as fatigue, the previous drink, (and it only has to be one), and the possibility of time change, working in GMT instead of 'local' time etc. are all factors that will effect the accuracy of a mental calculation. Even Einstein made errors in calculations. Washington made an error and he is paying for it, he has never claimed to be perfect.