Loot percentage mechanics ?

When a party kills a boss, he drops a chest of loot. My question hereby is: When do loot percentages come into play? Is the combined loot buff percentage of all members calculated somehow as the chest drop happens, or does the loot "appear" the moment someone opens the chest?

This would influence if just someone opens the chest, or the person with the highest loot buff and I've seen many discussions around it. Does anyone know which way it really is?

Thanks in advance