Jeez, and I thought mental maths was supposed to make things simpler and easier... some of the stuff in this thread makes my brain hurt
Clock code for xwind, TOD, RoD etc., sure, but fuel conversions, planning etc., I'll stick with a calculator thanks - why make things harder than they have to be??
I use my head for fuel quite a lot- but its more for the 'get an idea' rather than gospel.
Its not that hard to remember that 100lbs of JetA is almost 60 litres.
For example if i want 500lbs ill be ordering about 300 litres, and if its been ordered for me if all of a sudden i saw 400l or 200l I know something somewhere isnt quite right for whatever reason.
That's not "harder than they have to be". It's actually quicker than removing calc/phone from pocket and punching in 500 / 1.76 = into it!
Sure we arent trying to sin/cos/tan angles or anything like that so some things are irrelevant, but mental maths is very useful to dummy-check figures in all aspects of flying or at least get a rough estimate to get things going and work out an absolute later.