It's all about thinking on your feet and rates.
1 mile per minute covered at 60 knots.
2 miles per minute covered at 120 knots.
That makes it easy to calculate distances.
200 miles = 100 minutes at 120 knots. Easy.
200 miles = 200 minutes at 60 knots. Easy.
Based on those figures, it's very easy to work out how far you will travel at 90 knots - it's 150 minutes. See?
Same with fuel burn. You need to check your calculated figures by doing them in your head.
60 litres per hour. 5 hour flight, therefore 350 litres, plus reserve.
Then calculate your reserve such as 45 minutes fuel, therefore 45 litres.
Thinking quickly and checking calculations by using your head.
Then try it in your head up high where the air is thin!