(from what I understand it might be a number for instance 2 stun = 1 physical but right now I'm doing 1=1). If you go over physical you enter overflow, if the overflow equals the amount of bodies that person has then they are dead.
Overflow is actually listed in the rules and I'm afraid you're doing it wrong.
Page 170:
"For
every two full boxes of excess Stun damage, carry over
1 box to the Physical damage track"
"If a character takes more Physical damage than he has boxes in the Physical damage track, the character is in trouble. Overflowing the Physical damage track means he’s near death. Instant death occurs only if damage overflows the Physical damage track by
more than the character’s Body attribute."
"If no medical attention is available, the character takes an additional box of damage from loss of blood, shock, or other complications for every (Body) minutes he manages to hang on."