With a mutli level IF statements it can become very complex and difficult to debug. I suggest you use a Select Case Statement.
my_value is the value that you are reading, new_value is your return value
I assume the numbers are integers, if they are string they need to enclosed in ""
Select Case my_value
Depending on the specific circumstances of course, I would skip the code altogether and put this into a table. If you have these numbers in a table already then this letter thingy should really be considered an attribute of the entity. Apart from owt else - it means you do not need to rewrite the code everywhere you need it or if you migrate your FE to another platform.
I would be tempted to use a table to resolve your problem. If you create a Table with 0-14 records and their classification there is no need of a if or case. If 15 is then used you simply add it to your Table.