This is an area that interests me. We run a lot of rebate programs for vendors so looking at it from the other side. Their challenge is getting the data in from Distribution in a consistent format to offer rebates through to T2 partners.
I would say you'd need to include eligible products/services in your model as Account A may have create more sales than Account B but Account B may be selling more of the products eligible for rebate. Unless you only work with vendors that offer flat rate volume rebates on all sales regardless of product. This link on product would link Vendors to Account and allow you to roll this up.