range of undervoltage and overvoltage

What is the application? Is this a motor protection relay? Have you consulted the manufacturer's website and/or performed a search of the internet for your answer?

variation should be between + or - 12% of 400V

Power company generally indicates what would be their %voltage flactuations (plus/Minus) of their system supply voltage. Now if your load side equipment require not to go below or above certain voltage (say no more than 415V for example for a costly equipment) to work with their 100% efficiency, then your over voltage should be set not more than 15x100/400=3.75%. Same way you may decide under voltage set point.