One thing you need to keep in mind is that your calculation will only tell you the temperature rise from current. You need to add to that the heat that will come from the components themselves. Even if the components have heatsinks, there will still be some heat flowing into the PCB through their leads, as well as conducted and radiated from the bodies/heatsinks.
To answer your question though, you want to use all of the available space for copper when you have power currents flowing and components that generate heat.