There is no way to adjust the EFIS's attitude indication to level when it is mounted with a slant. Also, if the EFIS is not mounted straight, the attitude, slip/skid ball, turn rate, and G-meter performance will suffer as well. In other words, the EFIS really needs to be mounted level with respect to roll (and yaw) to perform as advertised. The only adjustment possible is in pitch to accommodate panel tilt.