LONG WINDED but covers questions and problems I have seen here many times. There are many ways to skin a cat but think this is a good baseline.
A coolant system should have the following attributes to allow trouble free operation and should be a closed system. The engine coolant should be able to reach and maintain a minimum of 12 psi to maximize heat transfer. A radiator cap should be located at the highest point of the system to allow coolant to be added. A radiator cap and system must be capable of remaining sealed until pressure is above average system pressure by about 15%. The coolant must reach 12 PSI to reach maximum efficiency but too high of a pressures will exceed other abilities of average stock components like the gaskets sealing coolants and soft plugs or freeze plugs. I would say 20 psi is max for average stock engines. 20 PSI can be exceeded in racing preped engines and those with higher quality intake and head gaskets but I am trying to describe stock or average conditions. To make sure maximum safe pressures are not exceeded a release pressure is preset/built into the radiator cap from 14 to about 18 PSI depending what you buy. When this pre-determined max level is reached the cap relieves pressure and fluid from coolant system dumps (into a recovery tank or expansion tank to be sucked back into engine as it cools.
Radiator can be mounted lower than engine but will need a way to add coolant to system at a level higher than system (engine and rad.) this is usually a remote expansion tank. The expansion tank must be plumbed so that the in and out hose back to system is covered with coolant or in the bottom of tank so air is not drawn back into and mixed with coolant in the engine. Tank can have air in top of tank for expansion but can not get mixed with circulating coolant. The remote expansion tank needs a Rad. cap so it can relieve max pressure if needed but air gap on top will compress unlike fluid to allow cold to hot volume changes. If capacities are correct you do not need a recovery tank on the expansion tank. I recomend an 18 psi cap on your tank. 20/ 23 or more PSI if a must and only if everything is built to hold high presures. Pressures this high are a fix or trade off to other limitations in the system.
If engine gets hot while at cruise your system is too small. If it only gets hot when in stop and go your system may be marginal size but lacks enough air flow to work and need more air flow and/or ways to expell heat like
Oil coolers and more fans. All this assumes there are no Engine or outside problems causing the symptoms of overheating.