When I worked for the Comptroller of a large (fortune 100 size) reinsurance company, I ran the accounts payable and treasury department and one of my functions was to "build" the corporate budget. Each division would come to me and I would report all their previous years expenditures by line item (travel, salary, health care, office supplies etc). The division head would then use the guidance he was give - example - flat budget for next year, and then see what needed to be reduced or dropped to meet that goal, or given the other objectives for the coming year (expanded market penetration, or a new product launch), make the case for why his division need more not less in the budget in the coming year. When it got to the Comptroller, I provided only a summary by division at first, showing if we were or were not meeting the budget goal, and why or why not by division. He would then ask for the details he needed to decide if those below him had made acceptable decision.
The other part of the puzzle was revenue projections, which I think is the missing part in Federal Gov. Budgeting. They focus on the expenditures WANTED and then go find a way to tax more to raise revenue. That is an option a private party does not have - command revenue growth