I found a problem were my calculations doesn't match with geometric drawing. I have an object placed at a position half of the focal legth of a concave mirror. I had drawn an image similar to one given below (Rays from center of curveture and parallel to principal axis) (like said in this question) but the image was not what it was supposed to be (They intersected much nearer to the mirror than they should). I tried with rays from focal point (reflacetd parallel to center) and parallel to principal axis (reflected through focus) but no avail.
But when I drawn this (found in wikipedia), it was correct.
Why my first two diagram didn't support the formula (1/f = 1/u + 1/v) ? Is drawing ray diagram from focal point (reflacetd parallel to center) and parallel to principal axis (reflected through focus) and the other method is wrong ?