With some help from a colleague who knows far more about calculations than me, I have attached some step by step guidance below on how to implement some quite clever design changes. The example business requirement which this covers is:
"I want to be able to add a number of different members of staff who are leaving as a collection on my incident and on each I will set their leaving date. When all of the leaving dates associated to the incident have passed, I want my incident to automatically move to the next status in my process".
In the example process design attached, I have obviously used actions that wouldn't ordinarily be in your process and made the process design very simple. This is just to show you the theory behind how this works so you can then use actions and the status within your process which makes the most sense to you.