I’m not exactly sure from your post what you need exactly, but I’m pretty sure you can use SQL Windowing with a Calc field something like this to get there:
SUM(PartBin.OnHandQty) Over(PARTITION PartBin.BinNum ORDER BY PartBin.BinNum)
Perhaps you can clarify what you are after?
I did a blog post on this type of thing, and it provides a bit more complex example of Aggregates Over… also known as Windowing: