A colleague helped me figure this one out.
Step one: RMC on Bill of Materials Anchor under Sheet Format, under Sheet, and selecting Set Anchor gives you the opportunity to choose your anchor point on the sheet by clicking it (first pic).
Step two: When creating the BOM you need to select the table position you want (second pic).
Now, no matter how many items I add or which way I move my column sizes the whole table stays put.
Except when you start splitting tables.
Then they reaquire their habit of galloping around the screen