The rate of $15000 per ton is an assumption made on the basis of the conference call conducted by HEG mgmt in the following link. (Page 21 has some details)
With respect to Tokai Carbon, their realisations on the basis of your calculations are quite low. While their website mentions annual capacity of 96k tons, I suspect capacity utilisation is lower than 90%.
I was not able to find details on how many tons they have sold, however, I did get some data on the price realisation as of Dec 2017 in the link below (which should have been realised in Q2 and later)
32-28 inch 24-18 inch
920,000 Japanese Yen / metric ton 900,000 Japanese Yen / metric ton
16 inch and below 1,300,000 Japanese Yen / metric ton Shortening of payment term
The above works out to $8275 and $11700 per ton but the data is dated. This also leads me to believe that capacity utilisation is lower than 90%.