I think you are simply looking to define the button with "GoToRelatedRecord", then defining the layout to use in the GTRR dialog.
Your calc fields can be part of the related record in your child table so no copying of data is needed.
Simply make your new layout and calc fields, define your button as GTRR and see if that gets you what you're after.
I tried that but it seemed to always go to the related record from the first portal row. Perhaps I've set something else up incorrectly?
Got it! Many thanks!