With regular usage I was always much faster on the whizzwheel than with a Calculator, and now I have to re-learn it 17 years later it is all coming back very quickly. For ease of use, I marked mine with pairs of different coloured arrows for Fuel Burm, speed, etc and then could just line up the arrows instantly and read.
For instance: if we were asked to divert to an accident and I needed to check we had the range to pick up the wounded and take them to Hospital, I could check the guages for fuel content, line up the orange arrows for fuel burn rate and read off the flying time to min fuel, then line up the green arrows for speed and read off what distance we could cover. Much quicker than typing it into a Calculator! (and typing this!)