Header tank is part of the closed system, with a 50 kPa (about 7 psi) pressure cap
Given that the engine is never near boiling, I do not think the pressure cap even matters.
I did notice that when I took the cap off this morning we had an immediate overflow, including some air bubbles.
Header tank was about bathtub temperature, say 40 C, although the engine was up to its normal operating temp in the 70's

We determined capacity by adding my calculations of the volume of hoses to the water heater and cabin heater to the engine vendors stated engine coolant capacity

We just noticed that we can hear water running at the electric pump that runs the cabin heater loop. That suggests air leaking in to me. The loop is about 50 feet of 5/8" hose wandering through the boat. Water flow is 3 gpm +, so I reckon fast enough to push air pockets out. The heater works well, suggesting good water flow.

We cannot find any water leaks out, but plan to go around the system again.