How to display results calculated from other DFFs in DFF.

My customer wants the DFF to default to displaying results calculated from other DFFs.

For example, I would like to do something like display the result of the contract amount - (DFF1+DFF2) in DFF3.

So I tried the following DFF settings, but they did not work.

Default Value:

SELECT
ESTIMATED_AMOUNT - (ATTRIBUTE_NUMBER1 + ATTRIBUTE_NUMBER2)
FROM
OKC_K_HEADERS_ALL_B

When I checked within Customer Connect, I found a discussion that this kind of setup is not possible, is this still the case today?

If anyone knows, please let me know.

Thanks,

Mayuko