It's one of the most important and obvious questions from stakeholders - and quite difficult to answer accurately.
We use the Monte Carlo simulation to predict how far down the backlog a team(s) will get for a specified budget or deadline.
The Monte Carlo simulation uses the actual behaviour of the cards across the board, specifically for the current team and the project (or release) they are working on.
The model predicts the 'Probable' and 'Possible' deliverable scope, which helps the business focus on the marginal scope.
There is not much point in focusing on scope around here (1) as this will almost certainly be delivered. Similarly, don't spent too much time talking about scope down here (2) as this is almost certainly not going to be delivered.
Instead everyone should be focused on (3) the scope that *could still be delivered - if the team and 'the business' work through the priorities and manage to improve the throughput rates.
The model can be run every few days and adjust for actual changes in the project, like newly added scope or an increase in the amount of rework.