Quick numbers checks using what some have mentioned here are fine under certain conditions but it would be very irresponsible of any pilot not to have a calculator at hand at all times. Under pressure or duress the brain can be very unreliable & simply wrong so always have two options for working out any calculations. Why else do we have such high levels of automation these days even in basic A/C, 'cause it takes the 'guess' work out of a lot of flying & that includes number crunching using the brain only.End result safer!
When I was taught to drive a plane everything revolved around a whiz wheel as well as being able to do simple maths in yr head but a calculator was always there close at hand.
Simply being alert at all times & having good situational awareness means brain number crunching is kept to a minimum.
Wmk2