FIELD based filter in FlowField (Dates)

So I have a table with 3 fields:

- Start Date

- End Date

- Amount (FlowField)

I would like the Amount flowfield to be "SUM(Amount) from "Sales Lines" where "Posting Date" between "Start Date" and "End Date"

I would like to enable a Page so a user can create entries in this table, and on the spot be able to see the Amount.

Seems like when setting FILTER in a flowfield's definition, you cannot use field names. 

Also when using FIELD, we cannot user formulas

Any workarounds to achieve this??