How can we establish lead time between two production lines?

Summary:

We are using flow schedules and have two lines using the same PS plan, line 1 Sub-assy line 2 Finished goods. We would like to achieve a lead-time of 180 mins between sub-assembly & FG lines. We've tried adding preprocessing lead time on the planning attribute of the sub-assembly item, however we don't see this working on the PS plan. We would like to understand how PS plan pegs items from one work order to other. In the attached screenshot we could move the flow schedule of the FG start time before the sub-assembly flow schedule completion time.

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