Safety stock

Hi sir..hope u r good at health.plz help me with following. I knw that;

Minimimum inventry level=re-order pint-(avg usage × avg lead time)

And
Avg usage= max usage + min usage/2

But my queation is, if there are more lets say 6 different usage levels are given for example..600, 900, 1200, 1500, 1800, 2000 then still we should choose minimum usage and maximum usage to calculate avg usage or we will will sum up these all and will divide it on 6?