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."
"And do you think (fop that I am) that I could be the Scarlet Pumpernickel?" -- Looney Tunes, The Scarlet Pumpernickel (1950, Chuck Jones)