Independent Demand for Purchaed Parts

How do you drive demand using MRP for purchased parts that are used on
manufacutred goods and are sold independently? We have several raw
materials that we buy to use on our products but also sell separately
to customers. Our consultants recommended using the forecast module
for these parts, but it is my understanding from them that we would
have to adjust our forecast by our on-hand inventory each month in
order to receive the suggestion we wanted. This seems impractical.
I'm wondering, are there any other ways that people are driving
independent demand for purchased parts without using the forecast