the is null part is working.
the second part I need is that I want the data in the Related Customer (Referrals Only) - Lead - District to be the "default". Essentially I want:
[Related Customer (Referrals Only) - Lead - District]
OR
If(not IsNull([Related Customer (Referrals Only) - Lead - District]),[Related Customer (Referrals Only) - Lead - District],[District])