I am not so sure it's that simple.
As a man who negotiates for a living, it's a safe bet Boras doesn't actually ask for what he wants, as that would weaken his position, having nothing to give up.
He presumably has a figure in mind, and then asks for X% more, giving him some wiggle room to accommodate both sides.
The question then becomes - what is that X%? That number ultimately comes from Martinez, who is the guy with the actual right to approve or reject any offer. Just because he is taking Boras' advice (which he is paying handsomely for) doesn't change this.