I saw this request come up again on the Salesforce Community last week and thought that there must be a way to solve it. This is something I see all the time and is a common request.
I believe that much of what I’ve done here can be completed using the Process Builder but I decided to use Andy Fawcett’s Declarative Lookup Rollup Summaries instead because as of right now that’s what I’m more comfortable with. For those that want to try this with Process Builder and Flows, take a look at Michael Gill’s solution and see if you can get a hybrid version going. Hopefully I’ll find some more time soon and can take a stab at that as a follow-up post to bring the solution completely in-house