um, basically, looking at the wheel from the side, at is face if you will.
Imagine a line vertically through it, with equal halfs.
Then quater it so you have a horizontal line. Not, where they cross is the centre right, thats also the centre of the steering pin. Naturally, you would think it would steer along the axis of the vertical line, but it dont work like that. No cars have a O deg castor as it makes seteering very, well ineffective! so, you lean the axis or orientation of the pin top and bottom (whilst keeping the centre of teh pin in teh same place) back. Then continue the line of the steering pin to the ground intersect and the amount in deg from the ground intersect verticle and the stering pin axis is the castor....
might be easier if i draw it! lol