Nested IF Condition Excel 2010

Try below formula
=IF(A1="","",LOOKUP(A1,{0,0.8,0.9},{0.5,0.75,1}))

I assume that is a typo for C1 to be 100 percent rather you would like B1 to reflect this...

To keep it to a basic Nested IF...

=IF(A1>=.9,1,IF(A1>=.8,.75,.5))

This says IF A1 is Greater than or Equal to 90% Then it is 100%, Otherwise IF A1 is Greater Than or Equal to 80% then it is 75% Otherwise "ELSE" it is 50%.

By nesting the IFs as you asked and starting with one end or the other we are able to call on the next top/bottom of tier as the previous tier will have already filtered out those that meet its criteria.

Few things - You have gaps in your ratings. What you have asked is that if the value of A1 is ABOVE 90%, Between 81% AND 89% OR LESS than 80%.... This means that you have left everything from 80.00% Through 80.99%, 89.01% - 90% completely in the dark and at risk of being an error if you do not have a catch, but if you are not careful your catch all could be a lie for someone that should be in a bucket...

I fixed it in the above example simply with >= that equals sign as well as shifting the exact point in ref to each tier.