the radiator should always be full. The overflow bottle is an expansion chamber basically when the coolant is hot, it expands and some is pushed to the overflow bottle and then taken back into the radiator once cool.
what you need to do, is fill the rad with bleed screw on the manifold taken out til the system is full. then reseal the system, make sure the overflow bottle is filled to the MAX line and then mark it.
drive for a few days and then re check it.
if you have lost coolant from either the rad or the overflow then you either have a leak or a gasket problem, simple as that