Overpitching a healthy culture is more likely to result in overattenuation than underattenuation. The place where overpitching hurts is in future generations. Overpitching as a practice causes the average age of the cells in the culture to rise over time, resulting in declining performance.
If you decide to crop from this batch, crop between 100 and 150 milliliters of thick slurry. A thick slurry that contains 40% yeast has around 1.2 billion cells per milliliter. Contrary to what the yeast calculators tell you, 120 billion viable cells will attenuate a 5-gallon batch of well-aerated wort up to 1.060s without breaking a sweat. In fact, if one's sanitation practices are solid, 60 billion viable cells will do the job.