If Coolant Overflow System Leaking, How Low Will Coolant Go?

[shertex]

As I have learned from a number of you, my car (1992 300D) has a coolant overflow reservoir. Overflow coolant goes into this tank when hot, then is sucked back into the the expansion tank when the system cools.

I strongly suspect that this system (either hose or tank) is leaking and that that accounts for my loss of coolant. The reason I suspect this is that the loss rate has substantially decreased as the coolant level when cold has approached the seam in the expansion tank. Currently the level looks to be about 1/8" inch below the seam.

My question: how low would the cold level have to get before I could conclude that the overflow system is NOT the problem?

Thanks.