the easy answer is that for a straight extrusion, it'll be slightly *above* the midpoint of the total span (above because the masthead structure adds some mass up high)
For a tapered extrusion it'll be lower. But if the question is about where to put slings on the mast when it is being pulled from the boat, the safe answer is to have the slings above the midpoint.
And, yeah, for a double-spreader rig that's usually just above the lower spreaders. (don't forget to consider the below-deck parts of a keel-stepped mast in figuring the total length...)