Mimic allocation wizard rule via Formula based rule with fixed amount as Source

Summary:

We are trying to mimic an allocation rule which is created through allocation wizard currently to Formula option having source as fixed amount.

Can anyone help me in this scenario as I'm are not able to mimic this in formula.

Content (please ensure you mask any confidential information):

Currently in our case we have 7 segments and below is the scenario which we are trying to mimic in Formula rule . For eg.

Source: Fixed Amount

Basis:

Segment1: 9ABC (child)

Segment2: WXYZ (parent)

Segment3: MNOPQRS (parent)

Target:

Segment1: 9ABC (child)

Segment2: WXYZ (parent - same as Basis in this case)

Segment3: 1234567 (child)