Yep, ustored/unindexed fields can only be used as keys in the parent table and result in "one-way" relationships. (From the parent table you can reference records in the child table, but you can't use such a relationship to reference parent records from the child table.)
You will have to design a work-around. Either you implement a method that copies the data into a stored/indexed data field or you come up with a design that doesn't need the calculation. Keep in mind that since filemaker relationships can use operators other than = and can be defined with multiple fields, the calculation you couldn't use may not be needed.
If you have to physically copy the unstored calculation result, to a stored data field, you'll have to also design code that automatically updates this field when the unstored calc returns a different value. Depending on the design/purpose of your database this can be either a very simple or nearly impossible task.