How is available loot calculated?
Available loot is calculated by taking a percent of what the opponent currently has stored in their Markets, Mills, Farms and Caravans.
 
 
What is a loot bonus and how can I get it?
Loot bonuses increase the amount gained from Multiplayer Battles. You can acquire Loot Bonuses by purchasing the Loot Blessing via the Temple or selecting the British Nation.