Step 10: Take a reading
Make sure you are not in direct sunlight. This can significantly alter the value displayed (temperature and pressure are closely entwined).
Make sure the tin is leaning towards the markings (it has an ability to lean slightly in any direction, make sure that it`s always the same when you take a reading, give the beans tin a few gentle flicks to free up the beer can from the beans tins inner surface.
Level your eyes with the rim of the beans tin. So that the top looks exactly straight (not like an oval or a curve but absolutely dead level). Now count the markings from the top.
The higher it floats the lower the pressure, the lower it floats, the higher the pressure.
Why not try making regular recordings? Notice any correlations in readings and weather? If so, please leave comments on this page, it would be cool to hear if it works well enough to be useful! If it doesn't work, please do say so!