@PLoBello - while I can't answer your question directly, did the IRS calculated the penalty as if the income from the large August Sale occured evenly over the course of the year? If you were to re-do Form 2210 on your own to support your case for a lower penalty, be sure to check BOX C in Part II and then complete schedule AI (which is the last page of the link below). Schedule AI will allow you to put the large same into the 3rd quartile of the year and eliminate it from the first half of the year.
https://www.irs.gov/pub/irs-pdf/f2210.pdf
