From Power Pivot and Power BI:
The Golden Rules of DAX
Rule A: DAX measures are calculated against the source data.
Rule B: Each measure cell is calculated independently.
Rule C: DAX measures are calculated in logical steps.
Step 1: Detect pivot coordinates
Step 2: CALCULATE () alters filter context
Step 3: Apply those filter coordinates to the underlying table
Step 4: Filters follow the relationship(s)
Step 5: Evaluate the arithmetic
Step 6: Return the result
From Learn to Write Dax:
Place the tables in a star schema, and arrange them with the fact tables on top, and the data table below as a visual reminder that filters flow down hill, unless we use CALCULATE().
From Learn to Write Dax:
When there are two data tables such as Budget and Sales do not join them directly. Instead use intermediary tables, and make sure the filters come from the correct tables.
Nathaniel Cohen 