First you need to position the tank and 'cold feed' connection into your system.
Then calculate the height of your system above the expansion tank (the static head)
Lets say its 10m for arguments sake.
Your expansion vessel shoule be pressurised (assuming that its a diaphragm type) to a pressure equating to the static head. So the gauge pressure in the vessel should be 1Bar (G).(equal to 10m static water head)
Now that will create a balance between the water in your system and the pressure within your tank. The net result is that water will/should not enter your vessel at cold fill conditions.
This means that the whole of the expansion vessel is still available to let expanded system water in.
If you under pressurise the tank i.e. set the pre-charge pressure to 1.5bar or 5m head, then when you fill the system, water will enter your expansion tank and use up some of the valuable space that is needed to accomodate your expansion water.---When eventually the system water expands, the vessel will over-pressurise and might set off your safety valve etc. and lead to system water loss (and then you would need to refill the system and re-treat the water etc)
On the other hand, if you over pressurise the vessel at the commissioning stage, the overall system will again over pressurise giving equally poor results.
In practice, we select a vessel pressure equal to the static plus a small margin. The margin chosen depends on the manufacturer you talk to but something like 0.2 to 0.3 bar should be ok.
