Case Predefined Field formula error

We currently using a formula field for the From Address that has been working fine - 

IF (Case.RecordType.Name="HCM","hcm@abc.com","tlm@abc.com")

We have another criteria that we need to add for if they are a Level 1 customer. This formula works if I create a custom formula field (where Level 1 is a checkbox). I am able to select the fields from the editing tool:

IF(Level_1__c ,"level1@abc.com",

        IF (RecordType.Name="HCM","hcm@abc.com","tlm@abc.com"))

However, if I try to use this formula on the predefined field of the FromAddress on a Case, I get an error ""Field Level_1__c may not be used in this type of formula".

I even tried creating a custom field called FromEmailAddress using the updated formula, then referencing that new custom field in the formula editor on the predefined field and got the same error about it cannot be used in this type of formula.

Any way around this?

Thanks in advance!