How would you solve this?

Based on some simple calculations, cell H11 has a value between 0 and
70. I would like I11 to show a text output depending on the value of
H11... For example a score of 55 would end up being 5b and a score of
27 would be 4c.

The following formula works
=IF(H11=57,"5a",IF(H11=54,"5b",IF(H11=50,"5c",I F(H11=42,"4a",IF(H11=34,"4b",IF(H11=26,"4c",IF( H11=21,"3a",IF(H11=17,"3b","<3b"))))))))

....but I know that I cannot expand it with any more IFs. I would like
to be able to return a blank cell if H11 was blank and have more
options if necessary in the future.

Any help really appreciated.