What is the syntax for operations between fields in different tables?

Dear Collective Wisdom,

I want to perform an arithmetic operation between two different fields. I would think that would be fairly straightforward: [field A] + [field B], or in my particular case, I want to subtract one from the other:[field A] - [field B]. But the fields are in different tables, so [table A.field A] - [table B.field B]. And so far my various guesses haven't yielded the correct formula. Most of my field names use multiple words, for example, "total expenses". Same with my table names, ex. "monthly estimate". 

TIA for any advice on how to format to get what I want.