If you set the Show Broadcast Safe Exception to on in Resolve you can see which parts of the image need adjustment. Turn on Broadcast Safe in Project Settings. We do a number of calculations to correctly represent the resultant composite image in the UI display and limits.
You can grade for P3 just by connecting a P3 projector, and more technically, using Resolve Color Management (RCM) you can select to transform your timeline color space to P3 output color space.