[joetapley]

When you say the pin should be on the centre shot, would that be with the string aligned to the centre of the limbs the pin is in line with the string.

Yes. You can also aim with the string picture running through the sight pin as this eliminates any variablility in head position from shot to shot. (in practice I've never found this to make any significant difference - in my case the string is lined up with the inside of the sight ring so minimal practical difference)