Always laughed at the time to decimal place card in the cockpit. Till I used it a few times... Oh how quickly we build reliance on something.
Seriously 0.1= 6 mins IS easy, but after 16 hr split duty have to confess, out comes the app, calculator and socks off...
Gone back to refusing to use tools and cards for simple arithmetic and low and behold, I can figure stoof out again. Amazing!
Even to the point that ops staff were simply amazed when I figured out the fuel in kg just by taking off 20%... wow no fms, calculator, app, whiz wheel or toes required.
So best tip, trick and shortcut? Put down the calculator! Pick up the pen!