I've made errors calculating my TOD before using my brain, how many times has my pocket calculator made them? 0. But a solid understanding and a mental check of the math confirms that.
Hmm, I've watched students use a calculator and get an answer one order of magnitude out.....
A mental check takes as much effort as rounding and getting the answer in the first place. And if you round the numbers (for most calculations that's all you need for aviation accuracy) and think about your process, you get that added gut feeling of reasonableness to boot.
The shortcuts as described here are not really that hard.
Personally, I think anyone who doesn't [want to] learn them is just lazy and unprofessional.
Capt Fathom
Metres Per second.... WTF? Do I need to know that?
Flown overseas recently?