Select 1 in your local z/OS character set. The character 1 is x'F1 in hexadecimal format.
Locate 1 in the code page ISO-8859 and find its corresponding hexadecimal value. This value is 31.
Locate offset x'31' in the code page file that is being defined. This offset location is at line 4, character 2, where the offset is relative to 1.
At that location, enter F1.