I definitely like your idea of running a trial.  I don't know how else you're going to know one-way-or-the-other if another fungicide application is warranted or not....  on your farm.  

Personally, I would use Veltyma.   Veltyma has a higher efficacy score for Tar Spot than Headline Amp.   I suggest 8.5 ounces of Veltyma with a surfactant, using at least 15 gallons of carrier.... if using a ground rig.


gordon