I just got my P515 recently, which I picked up for only about CAD $260 during the June sale (without chainrings).
I'm using a 1x oval chainring and had to adjust the slope from 100% to 97% after comparing the 3-second power readings with my Saris H3+ smart trainer.
Is anyone else using the P515 with a single-chainring setup? How have you found the power accuracy, especially when compared to using a traditional round chainring?