don’t know your whole setup but I can offer this:
use a light meter – takes the guess work out of camera settings for correct exposure at any shutter, ISO and aperture. Even more handy with a flash;
get the flash off camera – I use YoungNuo YN622C ETTL flash triggers. They’re brilliantly simple to use.
don’t use <P> – use manual mode with given aperture that you’ve pre-configured for your DOF, then use the light meter with a test flash to set Av and ISO and bam! You’ve nailed it. 🙂