This then that formula

You would do that using INDEX//MATCH.

=INDEX({1,2,3,4,5,6},MATCH(A1,{0,20,40,60,80,100,1000},1))

In this example, the value to be graded is in A1. The INDEX is {1,2,3,4,5,6}.  In its place you could have a vertical sheet range, like B1:B5 and, of course, these cells could contain anything, including text, and not necessarily in any particular sequence. You could use an array like {"perfect","very good","good","almost good","not good enough"}.

The MATCH function returns a number from the second array, above it's {0,20,40,60,80,100,1000}. Note the curly braces which signify an array but pay attention to the final 1 because that specifies the nature of the match. There are three choices: -1, 0 and 1. 0 (zero) requires an exact match. -1 will find a number that is not larger than look-up in an array that is sorted descending. Finally, the one we use above, 1 is finding a number that is not smaller than A1 in an array that's sorted in ascending order.

Accordingly, any number larger than 0 but smaller than 2 will be classified as "1". A number below 0 will result in an error as will any number from 1000 and up (you can increase the final integer to 10^10 if that's an issue. 20 will be the lowest number to return a 2.

You can play with the MATCH function until it does what you want. You might set the limit at 19.99 or 20.0001 or use a descending sort with -1 as match type. These are the things to watch out for.

1. Lowest number (that's why my array starts from 0)
2. Highest number (that's why my array extends beyond 100)
3. Exact matching number (that's why I chose type 1)

If these three come out correcty, so will all other variations.