Coolant overflow capacity?
If it covered the core by 1/2 to 1" it was fine.
The radiator will find its equilibrium point in short order.
Fill the rad up to the top.
Drive in hot weather, and the coolant will expand and some will be dumped out.
When it cools the rad will not be full to the top. DO NOT ADD COOLANT
If you keep doing this eventually the coolant will be at a point where no matter how hot it gets no more will be dumped.
If for some reason you keep dumping fluid after a week without refilling then you have some other things wrong like a leaking head gasket.
As to pressure cap...
You decide how much overhead you need
Define too low?
If it covered the core by 1/2 to 1" it was fine.
The radiator will find its equilibrium point in short order.
Fill the rad up to the top.
Drive in hot weather, and the coolant will expand and some will be dumped out.
When it cools the rad will not be full to the top. DO NOT ADD COOLANT
If you keep doing this eventually the coolant will be at a point where no matter how hot it gets no more will be dumped.
If for some reason you keep dumping fluid after a week without refilling then you have some other things wrong like a leaking head gasket.
As to pressure cap...
You decide how much overhead you need