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."
In seeking the unattainable, simplicity only gets in the way. -- Epigrams in Programming, ACM SIGPLAN Sept. 1982