Doesn't this work for you?

=IF(A1<74,"A",IF(A1<82,"B",IF(A1<89,"C",IF(A1<97,"D",IF(A1<104,"E",IF(A1<112,"F",IF(A1<119,"G",IF(A1<127,"H","I"))))))))

I've assumed that when A1=127 (which is not mentioned in your explanation), the formula should result in "I".