First, we will define your grades. For this, we will create a text grade type (explained here).
In our example it will look like this
The exact percentages are not important unless you are planning to calculate further averages with these grades.
Make sure the highest possible grade has a 100% and the lowest a 0%.
We will now create our grade conversion
Double tap on the header of the column holding the score > Grade type > Edit > Add new grade type > Grade conversion
You will now be able to configure the correlation between values and grades
With your grade conversion configured, you will now be able to assign it to our column.
It will look like this