Hi Thierry,
You can improve in a simple and effective way your undestanding of proportions and checking final results through:
1) Use measure approximation
2) Prepare some benchmark measures in advance

For instance, if you look into an excercise like the very popular ping-pong balls estimation, you can approximate shapes into simple ones (balls being cubes) and the plane being an even bigger cube. Then the exercise becomes as simple like finding how many cubes a rubik cube has. (Of course estimation needs more accuracy than this, but this is just showing the mental process to do it very quickly for double checking)
On checking soundness of data, if you have some benchmark measure in advance (e.g. per capita consumption of popular goods, population info etc) that helps a lot as well, especially in the market sizing questions. Feel free to connect if you need,

Best
Riccardo