Hi Ajinkya,
A possible soultion could be:
=IF(A1>100,"",IF(A1>90,"D",IF(A1>80,"C",IF(A1>70,"B",IF(A1="","","A")))))
I am sure those more skilled in Excel will have a much more elegant way to solve your issue, but this works.
I am not sure, but you may need to tweak the...