Proper coolant level?

[jay]

Forgive the cluelessness, but what should the proper coolant level be? I just popped a look around my engine, and my coolant level seems much lower vs. where I last remember it (last summer).

Temp-wise right now, it's around freezing outside, and the level is ~1/2 way b/w the "full" and "low" marks. Is this normal, or should I be adding more fluid? I have no idea where the fluid could've went, there are zero fluid leaks on the garage floor. I've been using the block heater for ~2.5hrs in the morning, all winter long - but I've been doing that since I got the car years ago.