In Rally, custom decimal fields created at the Task level do not automatically aggregate or roll up values to the parent User Story or Sprint levels. This behavior differs from standard fields like Task Estimate, which have native rollup functionality.
Rally SAAS
Native auto-calculation and rollup functionality is a core system-level feature specifically designed for standard Rally fields. This capability is not currently available for custom fields. Values for custom fields must be managed independently at each level.
Currently, there is no automated way to aggregate custom field values from Tasks to User Stories.