Hi Percy -
* * * UPDATE: IF RUNNING THE MODIFICATION SHOWN ABOVE * * *
Yes - that is correct. The Overflow (expansion) tank gets a regular pressure cap.
I install a "temporary" catch bottle when I first fill my system, and run it for a few hundred miles.
What I find is the system spits out water until it finds an equilibrium (about half way up the expansion tank) and then stabilizes. At that point I remove the catch bottle.
* * * If running a stock system, see LarryW's suggestion, below * * *
Rocky