Spells Duration Variables

[KhaN]

Im actually making spells, and there is one thing i dont understand, its spell duration formala, look like durations is calculated with 2 variables : "Buff Duration" and "Duration Formula". If i want make a buff of lets say 30min, what should i enter !? I just need to know how variables affect duration, because i have test alots of varibles, but i still dont get how it work ...

Thanks in advance :)