Creating a temporary table within a editable grid gallery

[MikeC1302]

Hello Community,

I have a use case that requires within a line of an editable grid gallery that I need to create a temporary table based on a multi select combo box holding a sharepoint list ID. The result from this then needs to be used to Lookup Lines within the same data source and pull the maximum end date. 
For those of you familiar with Microsoft project I am trying to recreate the Predecessors functionality based on the ID and end dates.

The powerapp solution will provide a solution for project planning multiple small (30-50 lines) projects. 

[MikeC1302]

I have managed to replicate this in excel as follows:

I hope this explains what I am trying to do.