Immediately after the Big Red Button has been pushed (by you or them) try hard not to engage in any unnecessary emotionally loaded banter with newly titled ex, if your goal is to discuss important details.
To be honest, it’s likely to be so highly charged, you are unlikely to get a great deal sensibly debated or organised.
It’s unfortunate when intricate, relationship-laced conversations descend into a verbal brawl, as they are generally never worth having at the back door of the relationship.
Be the bigger person in the room: put an end to the furore before it begins by not having it in the first place. Attempt to set up a more productive time to discuss any necessary questions or plans, and leave the room if necessary.