I think if you set your field properties on the calculated field to be like Part.Partnum, you should be able to set a filter on the BAQ as a primary browse. Then when you do the open with, it will populate that part number. I’d have to play with it to see if I could get it to work.
There might be more setup to get it to work. I end up making those with a little too much trial and error, and I’m not always sure what ends up working why, I just quit when it does.