This might not be exactly what you're looking for with fflogs but if you're on pc there are triggers you can use that will tell you in real-time if you hit your positional or not, if you're on console you're probably out of luck. Not sure if linking to them is allowed on the official forum so I'll refrain from doing so directly but you should be able to find it through google with "drg positional trigger" as the search terms.