@JustinC, I have been struggling with a very similar problem through this thread:
The commonality of our issues is Bubble’s limitation to only allow sorting on the RG’s thing (with some limited exceptions that are covered in the thread). The only difference is that you are looking to do a calculation, while I am trying to access fields in a child thing. Either way, I’d be happy using an expression to do the sort, but Bubble doesn’t allow… for now.