Here's a third answer. I was told by a Yanmar certified mechanic that the expansion tank normal level (between the hot and cold markings) should be level with the hose port on the heat exchanger (the hose that runs to the expansion tank). It can be higher if need be, but you have to add a 1/4 turn shut-off valve so you can check the coolant level under the cap (no you can't just look at the tank). He did not say it could be lower than the hose port.
I don't know what engine manufacturer you are talking about, but for a Yanmar it should be at or above the top of the heat exchanger.