Accounting : how to find the gross profit margin using the specific ID method?

It seems like you're on the right track. The only thing that I can see that you may be doing wrong is using "overall sales" instead of net sales. Were there any sales returns. If so, they need to be subtracted from gross sales. Also, if there were any freight-in charges or purchase discounts, they also have to be calculated into the cost of goods sold. Without seeing the actual problem, this is the best assistance I can give. Good luck.