Why is my digitiser uncalibrated after replacing it?

Iphone 3gs 16gb.

I replaced the digitiser and front glass (1 piece) put it back together, it detects my touches so is working, but thinks I am tapping about 1cm to the left of where I actually am tapping. Is there a way to calibrate the touchscreen like on many phones or have I done something wrong? I know this is a known issue, but there doesn't seem to be any up to date information or definite answer :( any help would be very appreciated as I'm doing it for a friend and I can't imagine them being 100% happy with this fix as only the outer glass was broken when they gave me the phone! thanks, max