I have a torpedo level. I set pitch to 6.5-7 depending on game, and obviously bias is always leveled to 0.00. Basically I go with factory recommendations, I prefer to play my games the way the designer intended them to be played.

Not only are phone apps not that accurate, many phones have a camera that sticks off the front now. I can't even lay my phone flush on a flat surface because the camera raises the top-left corner a bit over 1mm.