I have no take-off from the sub-marine tube...
With the standard Raceline configuration there is no need for at (as I understand it).
I added an 'expansion' tank just because I like running with the Raceline header always full.
With an expansion tank it is an adjunct to the cooling system, not an 'active' part of it, if that makes sense?
The lower tube on the expansion tank goes to the overflow from the thermostat housing.
(This is the small black hose in the first piccy tookaphotoof posted...)
Oh - edited to add, I obviously has a blanking cap on the thermostat housing!