Perfect Rolls

My friend and I were arguing about the statistics of getting a perfect roll on an item. If there is a certain maths method used can someone reply with it?
Also could you please tell me the odds of a perfect roll where there are 5 different stats all with the minimum 7 and the maximum 18.

 

You can see all the roll chances from the wynndata page of the item. For example, here's the page for Monster. You can find the rolls a bit down below.

 

if you want to know a bit more about how it works, each roll (except for sp, which is static) has a base number assigned to it. for positive values, the game rolls a value from 0.3-1.3, to the hundredths place (0.30, 0.31, 0.32 etc etc), and will round to the nearest whole number. for negative values, it rolls from 0.7 to 1.3, which the same method.