Automation to show amount of change in number column

We are using a finance board to track revenue - both invoiced and projected.

A revenue status column shows whether the value in the Revenue column is projected or invoiced (or ‘invoice ready for review’ etc). All revenue values go into dashboards that show all revenue for the year (including what’s invoiced and projected), so it’s important these stay in the same columns so I can show the data all together (I think).

When we go to invoice - the projected amount commonly changes.

What I would love is an automation to add a number to a new ‘change’ column which is the amount of change in the revenue, so that I can work out where we’re commonly over estimating vs. under estimating our revenues, and also report on what changed when the overall number changes significantly.

Revenue was $2000 with status projected
We changed revenue to $1000 with status invoiced
Change column would read -$1000 because the revenue went down $1000.