You are right. Arrive at the correct answer by multiplying the wrong value by another 1.5. Comes out to 830.4 tons of TNT. Which coincides with MichaelSmith's comment: "I saw another estimate of about a 1000 which is close enough."
I have yet to see any problem, however complicated, which, when you looked at it in the right way, did not become still more complicated. -- Poul Anderson