I would like to know where i can find (download) software where I can know what the percentage of an assignment accuracy and copying is?
Do you mean something with which you can detect plagiarism in a piece of written text..?
Yes exactly, plagiarism form the internet if students copy and pasted text form the internet to their assignments.
Google the suspected phrase(s)?
But that won't give me an accurate percentage to be fair with all students.
Life ain't fair. Catch one cheating and make an example of them. Many of us are subject to 'random' compliance testing.