in general there is recommendation of 0,3-0,5 bar on system top, which mostly depends on how precisely you can set your pressurization system.
pressurization serves only to prevent underpressure on the top of the system, which would suck the atmosphere air, the lower you set it the better, given that you are certain that underpressure will not occur. for smaller systems it is common practice to design to zero, means equal atmospheric pressure, and during start-up fitter will make just tiny increase in setting.