As you are well aware, budget outcomes in the past have deviated from projections - in some cases, significantly - and they will continue to do so. Accordingly, a well-designed set of mechanisms that facilitate midcourse corrections would ease the task of bringing the budget back into line when it goes off track.
In particular, you might want to require that existing programs be assessed regularly to verify that they continue to meet their stated purposes and cost projections. Measures that automatically take effect when costs for a particular spending program or tax provision exceed a specified threshold may prove useful as well.