Many people have claimed that the defense budget should be cut by this or that amount ($50 bn, $100 bn, $200 bn, $400 bn, or 10%), without specifying how to do it. (And the specifics proposed by most people who have specified their proposals would be disastrous for the military.)
By saying so, these people have proven that they know nothing about defense issues, including how the annual defense budget topline is determined. It isn’t determined by deciding to spend a fixed amount of money.
It is determined this way: firstly, all threats to the US are identified (state and nonstate actors alike; nuclear, conventional, and irregular threats); then, the DOD determines what missions must be executed to protect the US against these threats; and then, it determines what tools and what amount of resources it needs to execute these missions. The aggregate cost of all of these resources combined is the defense budget topline for a given FY.