The challenge that I have is that in one specific case, when a value in a different table = something I need to use a value other than FTE. Instead I need to use a calculated measure. Effectively, when the value in the other table is a specific value override [FTE] and use a measure instead. So in pseudo-code it needs to look something like this:
This will iterate over the table 'Data', and evaluate the IF statement for the value to use in the average for each row. You may still need to wrap this in CALCULATE(..., 'Budget'[FTE]<>BLANK()) syntax if your total is coming out wrong and [Measure] isn't used to replace blanks.
OK, I see now that this is because the budget value is in a different table. These tables are related so I believe I can create a calculated column lookup to make this work. Out of curiosity is there another/better way?