Hi Ambarrionuevo,
Are you using a default value for the calculation? If you, you can try using this formula:
substring("FailOK", 1 + 4 * not(../my:field1 = "0"), 4 * not(../my:field1 = ""))
You could also use a rule to set the value. On field1, under the Home tab, click Manage Rules, add an Action Rule which sets the target field to the Value: OK - Here you will need to add a condition (click the hyperlink underneath Condition) that this rule will only run if Field1 is not equal to 0. After this rule, add another action rule which sets the target field to the value: Fail - similar to the condition above, set the condition to only run if Field1 is equal to 0. I Hope this makes sense.
Regards,