It doesn't matter - The point is, by this time the tank is ready to cook-off, you can bail and fight with whatever tank commander kit you have (I'll recommend that be the only kit that allows tank healing at high health) - Or stay in and die like a true tanker
The idea is, at low damage, you can and should be able to repair damage (surface damge, You see the engineer kit showing an internal repair), at medium health, damage is permanent (say some mechanical damage) unless repaired from outside (health doesn't replenish) and at low health, the tank begins to take continual damage (internal fire) unless repaired from withn immediately, since you can't tell how long you've got....
At very low health, bleed is more rapid with external repair too slow to overcome bleed.. Would be nice to be able to drive with blackened vision outright at this point, showing 'out of control' damage i.e tank is functional but blind. At this point, either bail and live or keep fighting blind, more heroic considering the handicap, and die with the tank