A flat percentage for design development is a guess dressed as a calculation. Design management is what turns it into a measured position the client can interrogate.
A cost plan does not drift on its own. It drifts because design keeps moving after the number is submitted and nobody is measuring the movement against the basis that was priced.
A cost plan is a statement about design before it is a statement about money, and if nobody has written down which design was priced, the number cannot survive contact with the project.