What is the recommended pressure inside the expansion tank prior to charging the system with water?

The expansion tank is factory charged at 1.5 bar when empty. It can be charged with a maximum of up to a 6 bar. The formula is:

bar = 14.5 psig

P (Bar) = H (m) / 10.2 + 0.3 or P (psig) = H (ft) / 0.433

Charge vessel to calculated pressure before installing water/glycol mixture to loop.