Practical for moderate load swings and rates the two-element strategy can be used with boilers of any size. One element follows drum level while the other meters steam flow to provide a mass balance of the water in and out of the drum (Figure 3).
Hot Water Boiler Expansion Tanks - HVAC Hydronics #1
That is the balance point or pressure of the hot water boiler system hydronic loop. That is where little to no pressure change happens in the hydronic loop. It is also best to introduce the make-up water for the hot water boiler system and hydronic loop at a point close to the bladder expansion tank.