One out of four words is different so match is 75%. How do you arrive at 80?
It is not at all going to be easy if you want to take meanings to into account. And how would you match the sentences then? What should be the percentage for the following two sentences:
Manju is good guy
Manju is better guy
You will have to use a dictionary and define every possible match percentage based upon English grammar. This really tough job. All the best and if you implement this, let us know. :)