I think you're taking this thread waaaaaaaaay too seriously.
Personally, i think it's all a bit silly.
How far back do you go?
I was born in Wales, and so were every generation of my family since records have been kept. Am i 100% Welsh? The Romans invaded this area around 2000 years ago, and the Vikings around 1300 years ago and i'm sure they did some raping. Should we go further back to when homo sapien first left Africa 200,000 years ago... am i 1/100,000th Namibian and 1/78,000th Angolan?
I suppose the issue is that people just spout figures without any kind of definition.
If you're mother and father are both 1/2 English and 1/2 Italian... then are you also 1/2 english and 1/2 italian... or since your parents are american citizens and you were born in america, are you 1/3 america, 1/3 english and 1/3 italian? Do you not count the american part at all? If not, then what would you say to a person with an american mother and british father who claims to be half british and half american? If you don't count it at all, then why even acknowledge that your grandfather was german since maybe he only lived in germany and might well have been born to non-german parents! If you do count the american heritage, then what proportion of the pie do you give it?
Do you see the problems?
I know a guy who was born in Wales. His father is French and his mother is English. But his mother's parents were Italian and German. His father's parents were Albanian and Austrian. So what is this guy born in Wales? Is he Welsh? Or is he half english and half french. Or is he one quarter italian, 1/4 german, 1/4 albanian and 1/4 austrian because that's where his grandparents were from... so we'd be discounting his parent's nationality and the place he was born and raised...? Or do we go back further and see what nationality his great-grandparents were?