What you want to do is configure a linked record field from the [SalesReps] table to the [Master] table. (I assume each record in [Master] logs a discrete sale for a single sales rep.) Instead of using a single select or text field to specify sales rep in [Master], you’ll select the correct linked record.
After that, in [SalesReps] you’ll define a rollup field that follows the link back to [Master] and references the {RevenueMargin} field using an aggregation function of SUM(values). That will take the values from all of the {RevenueMargin} fields linked to Adam’s record — that is, the ones where {SalesRep} = 'Adam' — and add them together.
If your data structure isn’t quite as I imagine, let us know; there are ways to get at the data.
P.S. If your current sales rep names in [Master] are the same as the names in the primary field of [SalesReps], you can simply change the field type for {Master::SalesRep} from single-line text to linked record, and it will automagically create the links you need.
