Got to agree on mental maths.
Make it Happen Captain, agree with the mental stuff and whizz wheel at student level/flying school ops, however outside of GA, the calculator is your best pal.
Example..
If you do 8 sectors there is a "Deck log/ Flight log" to be filled out for each sector and it has to be crunched quickly, as the turn around can be 30 mins.. These compile of converting pounds to kg- litres to kg- minutes to dec. points, along with other calculations.
A calculator is a must, No guestimations here. It is also stated in Part B/SOP's, that a calculator is to be used for the Deck logs/MEL's.
ANOTHER THING GUYS/GIRLS.....GENERATION Y ARE A DIFFERENT BREED WITH REGARDS TO TECHNOLOGY....CALCULATORS RULE FOR THEM. I notice some here mentioned "batteries"....geez you must be a baby boomer or Gen x....
Solar power and battery combination along with pocket size/ compact units that fit right in your right pocket of your shirt,if you lose it, there is also a iPhone....
Anyhow, as mentioned still got to love mental maths for TOD points... Oops, the FMC does it for me....