I’m a little fuzzy on the exact table structure but you say you have used the “Is part of” feature. This creates a Parent/Child relationship between two tables. I’ll reply using assumptions and details to try to help best I can. So this is bit lengthy!
In your example I’ll call the understood Parent/Child tables - Table A and Table B. Table B is where you have these three columns you mentioned defined:
• Product #
• Planned time (Quantity * Standard time of the product)
I’ll then assume that you want the Sum of [Planned Time] to appear in Table A. I will also assume that you go THROUGH a Table A record Form view to be able to add the Table B record using its own Form view. (These assumptions are all important which I’ll explain below)
If all of these assumptions hold true and because of the “IsPartOf” switch, when you create the child records in Table B they are AUTOMATICALLY added to a Virtual Column list named by default as “Related Table B”.
Assuming the name of the summed column in Table A is [Total Planned Time], you can simply set the App Formula of this column to the expression `SUM([Related Table B][Planned Time])’ - note there is NO period in this expression.
Why All Those Assumptions Are Important
In order for the SUM to properly update (without any additional work), you MUST “touch” the Table A Form view AFTER adding/updating any of the Table B child records. This is so the SUM expression is activated to re-compute the value. In most cases this IS the normal flow so there are no issues. You would open the Table A record for edit, choose the Table B record to change, or add a new record. And when the Table B Form view is Saved, you are automatically sent back to the Table A Form view and all of its calculations are re-triggered, including the SUM.
So why do I bring it up?
Not all designs follow the intended flow. In those cases, custom work will be needed in order to get the SUM calculation to update when expected. That means we would need more details.
Please consider the above and if your structure seems to follow the default/intended flow, you are good to go. Let us know that this helped.
If not, post back and provide more insight to your implemented design and then we can help more from there.