First things first, set the scope so the corrector plate is facing the floor. Place the level on the back of the OTA and adjust the RA axis until the level reads 0 degrees. This particular level is accurate to 0.1 degree.
Next step is to measure the inclination of the wedge assembly.
Now place the level on the back housing in the same plane as it was on the RA housing. Adjust the declination axis until the level reads the same as it did when it was placed on the RA housing. Lock the declination axis and then adjust the setting circle to 90 degrees.