Are you able to calculate the total of a subset of the Product Line Items within an Opportunity, as opposed to only being able to include the total amount? The total amount calculates automatically, however I am needing to create a custom calculation
to drive Estimated Revenue, in which specific Product Line Items will be multiplied by the Contract Term, while other specific Product Line Items should be excluded from that calculation.

For example, if I have 3 Product Line Items in an Opportunity (A, B, and C)...I need to be able to calculate Estimate Revenue as ((A+C) x Contract Term) + B). Therefore, I want to be able to create two sub-totals of those line items (A+C)
and B. I'll then write a business rule / custom calculation to multiply the sum of (A+C) by the Contract term, then add that to the amount of B to get a "customized" estimated revenue that only applies a multiplying factor to a subset of
the Line Items. I'm just having difficult identifying a field / property on a Product Line items from which I can write that Business Logic.

Thanks in advance.