What I said was…"the act of Saving those entries will automatically trigger the Table B expression to recalc the Total Quantity. "
Not that I also said… “If you enter the Table B row in EDIT mode” - meaning you acces the Parent row through the Form View.
“Saving” being the key word here. The use case I outlined was entering the Parent table record FIRST and then selecting the Child record to edit OR the Add/New button to add a new Child record. Tap Save on the Child AND THEN you MUST tap Save on the Parent for all Child updates to be saved. This action WILL trigger any App Formulas on the Parent row. If the Total Quantity is implemented with an App Formula it will get updated.
There are scenarios where you can enter the Child record WITHOUT going through the Parent Form row first. Such as from the Detail View of the Parent row IF the child rows are being displayed. These use cases WILL NOT update the Parent row and some action will be needed to force the updates. I didn’t mention these because the question was about “best practice”. What I outlined is the best practice for that feature.