This calculation massively fails.
While juubi could be 1km tall, it isnt 1km wide and fat.
The said cube was filled only at the bottom some 10% if not less thus 100m (1000*1000*100)
You also made mistake while converting to joules and multiplied by higher mole number,
Your end result should be less by 915.15 putting fodders back to their place U_U
And i dont even want to start that magma element has heat on its own thus making sarutobi even more fodderistic.