say 80-90 percent overlap in day to day conversation. In literature, it variance is large. Once I tried reading a book of Urdu poetry(not contemporary) (shayari) by a famous writer, and I had to look up a dictionary on every fourth word :D.

I could imagine, similar or more intense case, only if I am reading a difficult Sanskrit text :D.

(Note: I am a native Hindi speaker)

Please give me an estimate of the percentage of overlap between Hindi and Urdu.

well I have a friend, she is half indian half pakistani ... she said that hindi and urdu is almost the same for everyday conversation but when reading more science and more literrary ( like studying stuff ^^)  things urdu has Arabic words and terms in it .. but generally it's the same, like it's  as if her mother and father are talking the same language there is no difference!

Hmm, good question. May someone did study on this topic. In Urdu, the numbers are same as in hindi. The grammar is also the same as.