We do.

Each stop of aperture lets in half as much or twice as much light. Each stop of shutter speed opens for half or twice as long.

The confusion arises when we use the same numbers on the aperture ring for our exposure times. The aperture f stop is derived from the diameter of the aperture opening whereas the actual area of the opening is proportional to the square of the aperture (a = pi x r squared).

Apertures are marked up in f stop figures, each one being the square root of two times the last one. e.g. 2, 2.8, 4, 5.6, 8, etc.

If we use the same numbers applied to exposure then we get half stop increments. If we use alternate numbers, they are full stops.

I think it's easier to avoid confusion and use 10, 20, 40, etc, as you are doing.

If you then find that the exposure you want is e.g. somewhere between 30 and 40 seconds then you can do a new test strip just between these values. With a bit more experience, you can judge where it is going to end up. e.g. if it's closer to 30 than 40 and just needs to be slightly darker, you might judge that 33 is enough and then you can be brave and do a whole print or just try an offcut of paper for that time.

Steve.