Everything coming from a secondary blended data source has to come as an aggregated value.
If you have only one row for the join-field, then SUM([Value]) will give you that value for each key value in your primary source.
So if you are blending on [Plan] and [Type], then SUM([secondary].[Factor]) = 0.75 for ABC-A in your underlying data table. (If there were two rows for ABC-A in the secondary source, it would sum the two [Factor] values together. If you don't want them summed, you could instead use MAX() or MIN() or AVG() instead of SUM.)
If you want, you could create a calc field in your primary source that does the SUM([secondary].[Factor], but it's not really necessary.