Someone once asked my why "Blog" when there is so much information already available. My immediate answer: My blog is my knowledge base and if I can help others by sharing, I am going to share. 

To include fields from a parent or N:1 referential related entity in a calculated field use the following format.

entityschemaname.fieldschemaname

(replacing Entity schema name with your Entity Schema Name and replacing Field Schema Name with your Field Schema Name)

so for example if your publisher is set to use the prefix "abc" and you had a custom entity called (abc_customentity) that the account referenced you could use the following to add a number from the custom entity and a number in the account. The key is that little DOT 

Account 

Action

Set account abc_wholenumbercalc to 

abc_customentity.abc_wholenumberfield + account.abc_wholenumberfield

(where account.abc_wholenumbercalc is a calculated field and abc_customentity is the Custom Entities Schema Name and abc_wholenumberfield is the custom field schema name in the custom entity and account is the schema name of account and abc_wholenumberfield is a custom field in account)