Assuming your Custom field is "ABN" (API Name = "ABN__c") then you can stuff this formula in a validation rule to ensure that ABNs entered are valid by using the same logic the government does .      not(and   (     LEN(TRIM( ABN__c )) == 11,     ISNUMBER(ABN__c),     mod(         (((VALUE(MID(ABN__c, 1, 1)) - 1 ) * 10) +      (VALUE(MID(ABN__c, 2, 1)) * 1) +      (VALUE(MID(ABN__c, 3, 1)) * 3) +      (VALUE(MID(ABN__c, 4, 1)) * 5) +      (VALUE(MID(ABN__c, 5, 1)) * 7) +      (VALUE(MID(ABN__c, 6, 1)) * 9) +      (VALUE(MID(ABN__c, 7, 1)) * 11) +      (VALUE(MID(ABN__c, 8, 1)) * 13) +      (VALUE(MID(ABN__c, 9, 1)) * 15) +      (VALUE(MID(ABN__c, 10, 1)) * 17) +      (VALUE(MID(ABN__c, 11, 1)) * 19)),     89     ) == 0   ))  
 
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