I am just wondering if there are ways how to calculate the maximum possible stats an item could give? As far as i understand only the minimum stats are shown.
If there is a formula for it then Iād like to know, thanks 
The item shows you the stats it has, not the minimum.
http://www.grimdawn.com/forums/showthread.php?t=25596 shows you min/max values
Use grimtools, click on Settings -> Item attributes -> Show min/max values.
Would be very nice if there was an in-game option ON/OFF for min max values also.