Exchanging air in basement
The best systems have a return in every room that has a supply register, but a lot of systems work just fine with only a central return (usually located right at the furnace). To achieve the highest efficiency, you're going to have to do a heat load calculation, however it sounds like it's working fine.

A good system will have balancing dampers placed throughout the duct work, but you can also balance the system by e.g., half closing all of the registers upstairs, to force more air into the basement.