Mapping Defects and 6 Sigma Levels in software

Hi,

I work for a software organization working towards six sigma. I want to know the mapping between defects found in software – measured in terms of Defects per Function Point and the six sigma level.
For example if our current defect rate is .002 defects per Function point, then what sigma level does this correspond to ?
Is there a document / standard that maps the defects in software to the sigma level.

Thanks in advance

Kalyani Sekhar