Can I point one line at another line's value?

A driver is budgeted for as many days as the camera crew, and a catering line follows the crew total. Type @ in any numeric field and a menu opens listing what you can point at, so the field follows that value instead of holding a number of its own. Change the source and the field moves with it.

Set a reference

Click into the field you want driven by another value. Empty fields show the placeholder Number or @.
Type @. The character is not inserted. It opens the menu.
The menu opens on Variables. Switch to Cost Items or Sections for the other targets.
On Cost Items, lines are grouped under their Section and carry the numbering you see in the budget. Hover a line to open its submenu, which lists Amount, Days, Fee, Total, Total Days and Total Cost Item under the heading Base, plus Prep Days and Wrap Days when that line has them. Total Cost Item is the line's base plus all of its AddOns.
On Sections, pick a Section to point at its total.
Click the row. The field becomes a chip showing the resolved value. Hover it to see the trail back to the source. Click the chip body to change the reference, or its X to clear it, which resets the field to 0.

Good to know

The menu lists the budget grouped by Section, so scroll the list to the line you want. When a value feeds more than two or three fields, make it a Variable: the Variables tab is short, and it is the tab the menu opens on.
A field on a Cost Item shows all three tabs: Variables, Cost Items and Sections.
The line you are editing is left out of the list, and so is any Section it sits inside, which keeps every reference pointing one way.
To drive a figure from the whole budget, such as a contingency, reference the Sections that make it up. Reference each chapter and the figure follows the whole.
Two fields that are set to follow each other both show 0. Clear one of them and the numbers return.
A reference follows its source through every change: a new day count, a new rate, a moved line.