Minecraft Forge - A pseudocode example of how to correctly calculate slot IDs in Containers - http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/modification-development/2743463-game-crashes-due-to-inventory

ContainerExample.java

SLOTS_PER_ROW = 10 // Constant field
	
ContainerExample() { // Constructor
	numRows = inventory.getSize() / SLOTS_PER_ROW; 
	
	for (row = 0; row < numRows; row++) {
		for (col = 0; col < SLOTS_PER_ROW; col++) {
  			addSlot(new Slot(inventory, col + row * SLOTS_PER_ROW, ...));
 		}
	}
}