When I meditate, I'm not sure if i'm focussing exactly at the centre of the body two finger breadths above the navel or not. How can i be certain?

It is not necessary to be too exacting when finding the point at the centre of the body. If you like, you can imagine the point at the centre of your body is enlarged so that it fills the whole of your stomach, that way you can be sure that you?re on target. In actual fact as soon as your mind comes to a standstill within this approximate area, it will automatically adjust to the exact centre required by itself.