S2k calculates fuel cost using WAC (Weighted Average Cost) also referred to as WAM (Weighted Average Margin). In the fuel inventory history you can see how this is calculated:
1- The old weighted cost per gallon
2- The new purchase cost per gallon
3- The new purchase gallons (based on purchased # on fuel invoice normally based on NET in most states)
4- The opening stick reading for that tank on day of delivery
5- The new total gallons after deliver
6- The new weighted cost per gallon
S2K takes the old cost (1) x gallons in ground at beginning of day of delivery (4) then gets the new cost (3) x new purchased gallons (3) and divides by the new TOTAL gallons after the delivery to get the new WAC (2)
($3.4194 x 12,057 = 41,227.7058) PLUS ($3.5336 x 8769 = 30986.1384) DIVIDED BY 20,826 = $3.4675