calculating amount of coal for turtles

I have a plot and I need to find out how much coal I need in order to fuel the turtle. I'm on a minecraft server (switchcraft) and my plot is 39*31 or 1209 blocks. I can have multiple turtles running. I know how to calculate, but I need someone to help me figure out the formula for it. Any help would be greatly appreciated.

The fuel calculation for any item is the number of items it would smelt * 10 so coal, which can smelt 8 items, gives 80 fuel to a turtle.